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, 3 insertions, 3 deletions

diff --git a/doc/languages-frameworks/python.md b/doc/languages-frameworks/python.md
index 82aeb112c93..48d9e0a0952 100644
--- a/doc/languages-frameworks/python.md
+++ b/doc/languages-frameworks/python.md
@@ -737,18 +737,18 @@ in (pkgs.python35.override {inherit packageOverrides;}).withPackages (ps: [ps.bl
 ```
 The requested package `blaze` depends on `pandas` which itself depends on `scipy`.
 
-If you want the whole of Nixpkgs to use your modifications, then you can use `pkgs.overridePackages`
+If you want the whole of Nixpkgs to use your modifications, then you can use `overlays`
 as explained in this manual. In the following example we build a `inkscape` using a different version of `numpy`.
 ```
 let
   pkgs = import <nixpkgs> {};
-  newpkgs = pkgs.overridePackages ( pkgsself: pkgssuper: {
+  newpkgs = import pkgs.path { overlays = [ (pkgsself: pkgssuper: {
     python27 = let
       packageOverrides = self: super: {
         numpy = super.numpy_1_10;
       };
     in pkgssuper.python27.override {inherit packageOverrides;};
-  } );
+  } ) ]; };
 in newpkgs.inkscape
 ```