Document where we can get metadata for nixpkgs

[pombredanne]

We have the .nix in nixpkgs, that are not static data:

• meta https://github.com/NixOS/nixpkgs/blob/623d10aa7a4499d63dbf5728c80da82b783c651d/pkgs/by-name/dj/djmount/package.nix#L34
• these are not easy to parser short of running nix proper https://github.com/NixOS/nixpkgs/blob/623d10aa7a4499d63dbf5728c80da82b783c651d/pkgs/shells/fish/babelfish.nix#L19
• fetchurl and more are interpreted at build time https://github.com/NixOS/nixpkgs/blob/623d10aa7a4499d63dbf5728c80da82b783c651d/pkgs/by-name/dj/djmount/package.nix#L12

So the .nix files are likely a dead-end.

Alternatively we can get may structured meta from the cache?