Add nixpkgs-lint

This is a simple tool to scan Nixpkgs for violations of the packaging
guidelines, such as multiple packages with the same name, packages
that lack a description or license, and so on.

To use:

$ nix-env -i nixpkgs-lint
$ cd .../nixpkgs
$ nixpkgs-lint

Current statistics:

Number of packages: 8666
Number of missing maintainers: 3711
Number of missing licenses: 6159
Number of missing descriptions: 1337
Number of bad descriptions: 633
Number of name collisions: 277