Add overlays mechanism to Nixpkgs.

This patch add a new argument to Nixpkgs default expression named "overlays".

By default, the value of the argument is either taken from the environment variable `NIXPKGS_OVERLAYS`,
or from the directory `~/.nixpkgs/overlays/`.  If the environment variable does not name a valid directory
then this mechanism would fallback on the home directory.  If the home directory does not exists it will
fallback on an empty list of overlays.

The overlays directory should contain the list of extra Nixpkgs stages which would be used to extend the
content of Nixpkgs, with additional set of packages.  The overlays, i-e directory, files, symbolic links
are used in alphabetical order.

The simplest overlay which extends Nixpkgs with nothing looks like:

```nix
self: super: {
}
```

More refined overlays can use `super` as the basis for building new packages, and `self` as a way to query
the final result of the fix-point.

An example of overlay which extends Nixpkgs with a small set of packages can be found at:
  https://github.com/nbp/nixpkgs-mozilla/blob/nixpkgs-overlay/moz-overlay.nix

To use this file, checkout the repository and add a symbolic link to
the `moz-overlay.nix` file in `~/.nixpkgs/overlays` directory.

1 files changed, 99 insertions, 0 deletions

diff --git a/doc/overlays.xml b/doc/overlays.xml
new file mode 100644
index 000000000000..d194c6bfc892
--- /dev/null
+++ b/doc/overlays.xml
@@ -0,0 +1,99 @@
+<chapter xmlns="http://docbook.org/ns/docbook"
+         xmlns:xlink="http://www.w3.org/1999/xlink"
+         xml:id="chap-overlays">
+
+<title>Overlays</title>
+
+<para>This chapter describes how to extend and change Nixpkgs content using
+overlays.  Overlays are used to add phases in the fix-point used by Nixpkgs
+to bind the dependencies of all packages.</para>
+
+<!--============================================================-->
+
+<section xml:id="sec-overlays-install">
+<title>Installing Overlays</title>
+
+<para>Overlays are looked for in the following order, the first valid one is
+considered, and all the rest are ignored:
+
+<orderedlist>
+
+  <listitem>
+
+    <para>As argument of the imported attribute set. When importing Nixpkgs,
+    the <varname>overlays</varname> attribute argument can be set to a list of
+    functions, which would be describe in <xref linkend="sec-overlays-layout"/>.</para>
+
+  </listitem>
+
+  <listitem>
+
+    <para>As a directory pointed by the environment variable named
+<varname>NIXPKGS_OVERLAYS</varname>. This directory can contain symbolic
+links to Nix expressions.
+
+  </listitem>
+
+  <listitem>
+
+    <para>As the directory located at
+<filename>~/.nixpkgs/overlays/</filename>. This directory can contain
+symbolic links to Nix expressions.
+
+  </listitem>
+
+</orderedlist>
+</para>
+
+<para>For the second and third option, the directory contains either
+directories providing a default.nix expression, or files, or symbolic links
+to the entry Nix expression of the overlay.  These Nix expressions are
+following the syntax described in <xref
+linkend="sec-overlays-layout"/>.</para>
+
+<para>To install an overlay, using the last option.  Clone the repository of
+the overlay, and add a symbolic link to it in the
+<filename>~/.nixpkgs/overlays/</filename> directory.</para>
+
+</section>
+
+<!--============================================================-->
+
+<section xml:id="sec-overlays-layout">
+<title>Overlays Layout</title>
+
+<para>An overlay is a Nix expression, which is a function which accepts 2
+arguments.</para>
+
+<programlisting>
+self: super:
+
+{
+  foo = super.foo.override { ... };
+  bar = import ./pkgs/bar {
+    inherit (self) stdenv fetchurl;
+    inherit (self) ninja crawl dwarf-fortress;
+  };
+}
+</programlisting>
+
+<para>The first argument, usualy named <varname>self</varname>, corresponds
+to the final package set. You should use this set to inherit all the
+dependencies needed by your package expression.</para>
+
+<para>The second argument, usualy named <varname>super</varname>,
+corresponds to the result of the evaluation of the previous stages of
+Nixpkgs, it does not contain any of the packages added by the current
+overlay nor any of the following overlays.  This set is used in to override
+existing packages, either by changing their dependencies or their
+recipes.</para>
+
+<para>The value returned by this function should be a set similar to
+<filename>pkgs/top-level/all-packages.nix</filename>, which contains either
+extra packages defined by the overlay, or which overwrite Nixpkgs packages
+with other custom defaults. This is similar to <xref
+linkend="sec-modify-via-packageOverrides"/>.</para>
+
+</section>
+
+</chapter>