nixpkgs manual: move contributing chapters into one folder

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+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xml:id="chap-quick-start">
+ <title>Quick Start to Adding a Package</title>
+ <para>
+  To add a package to Nixpkgs:
+  <orderedlist>
+   <listitem>
+    <para>
+     Checkout the Nixpkgs source tree:
+<screen>
+<prompt>$ </prompt>git clone https://github.com/NixOS/nixpkgs
+<prompt>$ </prompt>cd nixpkgs</screen>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     Find a good place in the Nixpkgs tree to add the Nix expression for your package. For instance, a library package typically goes into <filename>pkgs/development/libraries/<replaceable>pkgname</replaceable></filename>, while a web browser goes into <filename>pkgs/applications/networking/browsers/<replaceable>pkgname</replaceable></filename>. See <xref linkend="sec-organisation" /> for some hints on the tree organisation. Create a directory for your package, e.g.
+<screen>
+<prompt>$ </prompt>mkdir pkgs/development/libraries/libfoo</screen>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     In the package directory, create a Nix expression — a piece of code that describes how to build the package. In this case, it should be a <emphasis>function</emphasis> that is called with the package dependencies as arguments, and returns a build of the package in the Nix store. The expression should usually be called <filename>default.nix</filename>.
+<screen>
+<prompt>$ </prompt>emacs pkgs/development/libraries/libfoo/default.nix
+<prompt>$ </prompt>git add pkgs/development/libraries/libfoo/default.nix</screen>
+    </para>
+    <para>
+     You can have a look at the existing Nix expressions under <filename>pkgs/</filename> to see how it’s done. Here are some good ones:
+     <itemizedlist>
+      <listitem>
+       <para>
+        GNU Hello: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/misc/hello/default.nix"><filename>pkgs/applications/misc/hello/default.nix</filename></link>. Trivial package, which specifies some <varname>meta</varname> attributes which is good practice.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        GNU cpio: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/archivers/cpio/default.nix"><filename>pkgs/tools/archivers/cpio/default.nix</filename></link>. Also a simple package. The generic builder in <varname>stdenv</varname> does everything for you. It has no dependencies beyond <varname>stdenv</varname>.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        GNU Multiple Precision arithmetic library (GMP): <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/gmp/5.1.x.nix"><filename>pkgs/development/libraries/gmp/5.1.x.nix</filename></link>. Also done by the generic builder, but has a dependency on <varname>m4</varname>.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        Pan, a GTK-based newsreader: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/newsreaders/pan/default.nix"><filename>pkgs/applications/networking/newsreaders/pan/default.nix</filename></link>. Has an optional dependency on <varname>gtkspell</varname>, which is only built if <varname>spellCheck</varname> is <literal>true</literal>.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        Apache HTTPD: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/servers/http/apache-httpd/2.4.nix"><filename>pkgs/servers/http/apache-httpd/2.4.nix</filename></link>. A bunch of optional features, variable substitutions in the configure flags, a post-install hook, and miscellaneous hackery.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        Thunderbird: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/mailreaders/thunderbird/default.nix"><filename>pkgs/applications/networking/mailreaders/thunderbird/default.nix</filename></link>. Lots of dependencies.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        JDiskReport, a Java utility: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/misc/jdiskreport/default.nix"><filename>pkgs/tools/misc/jdiskreport/default.nix</filename></link> (and the <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/misc/jdiskreport/builder.sh">builder</link>). Nixpkgs doesn’t have a decent <varname>stdenv</varname> for Java yet so this is pretty ad-hoc.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        XML::Simple, a Perl module: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/perl-packages.nix"><filename>pkgs/top-level/perl-packages.nix</filename></link> (search for the <varname>XMLSimple</varname> attribute). Most Perl modules are so simple to build that they are defined directly in <filename>perl-packages.nix</filename>; no need to make a separate file for them.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        Adobe Reader: <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/misc/adobe-reader/default.nix"><filename>pkgs/applications/misc/adobe-reader/default.nix</filename></link>. Shows how binary-only packages can be supported. In particular the <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/misc/adobe-reader/builder.sh">builder</link> uses <command>patchelf</command> to set the RUNPATH and ELF interpreter of the executables so that the right libraries are found at runtime.
+       </para>
+      </listitem>
+     </itemizedlist>
+    </para>
+    <para>
+     Some notes:
+     <itemizedlist>
+      <listitem>
+       <para>
+        All <varname linkend="chap-meta">meta</varname> attributes are optional, but it’s still a good idea to provide at least the <varname>description</varname>, <varname>homepage</varname> and <varname
+          linkend="sec-meta-license">license</varname>.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        You can use <command>nix-prefetch-url</command> <replaceable>url</replaceable> to get the SHA-256 hash of source distributions. There are similar commands as <command>nix-prefetch-git</command> and <command>nix-prefetch-hg</command> available in <literal>nix-prefetch-scripts</literal> package.
+       </para>
+      </listitem>
+      <listitem>
+       <para>
+        A list of schemes for <literal>mirror://</literal> URLs can be found in <link
+          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/fetchurl/mirrors.nix"><filename>pkgs/build-support/fetchurl/mirrors.nix</filename></link>.
+       </para>
+      </listitem>
+     </itemizedlist>
+    </para>
+    <para>
+     The exact syntax and semantics of the Nix expression language, including the built-in function, are described in the Nix manual in the <link
+    xlink:href="http://hydra.nixos.org/job/nix/trunk/tarball/latest/download-by-type/doc/manual/#chap-writing-nix-expressions">chapter on writing Nix expressions</link>.
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     Add a call to the function defined in the previous step to <link
+    xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix"><filename>pkgs/top-level/all-packages.nix</filename></link> with some descriptive name for the variable, e.g. <varname>libfoo</varname>.
+<screen>
+<prompt>$ </prompt>emacs pkgs/top-level/all-packages.nix</screen>
+    </para>
+    <para>
+     The attributes in that file are sorted by category (like “Development / Libraries”) that more-or-less correspond to the directory structure of Nixpkgs, and then by attribute name.
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     To test whether the package builds, run the following command from the root of the nixpkgs source tree:
+<screen>
+<prompt>$ </prompt>nix-build -A libfoo</screen>
+     where <varname>libfoo</varname> should be the variable name defined in the previous step. You may want to add the flag <option>-K</option> to keep the temporary build directory in case something fails. If the build succeeds, a symlink <filename>./result</filename> to the package in the Nix store is created.
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     If you want to install the package into your profile (optional), do
+<screen>
+<prompt>$ </prompt>nix-env -f . -iA libfoo</screen>
+    </para>
+   </listitem>
+   <listitem>
+    <para>
+     Optionally commit the new package and open a pull request <link
+     xlink:href="https://github.com/NixOS/nixpkgs/pulls">to nixpkgs</link>, or use <link
+     xlink:href="https://discourse.nixos.org/t/about-the-patches-category/477"> the Patches category</link> on Discourse for sending a patch without a GitHub account.
+    </para>
+   </listitem>
+  </orderedlist>
+ </para>
+</chapter>