Add documentation for nixpkgs.config settings

[matthiasbeyer]

This PR is a follow-up PR for #5892

It documents the following settings:

• allowUnfree
• allowUnfreePredicate
• whitelistedLicenses
• blacklistedLicenses

Feel free to suggest more documentation at the appropriate places.

[domenkozar]

🍺 That's a good start. We have to document how it's used outside of NixOS.

So given a file ~/.nixpkgs/config.nix, you can declare:

{
  allowUnfree = true;
}

[jgeerds]

What are valid values for license?
[ "gpl3" ] or with stdenv.lib.licenses; [ gpl3 ] ?

[matthiasbeyer]

@iElectric , @geerds I justed pushed fixups for the stuff you mentioned.

[domenkozar]

Thanks, I'll take it from here :)

[matthiasbeyer]

I'd continue improving it, if you tell me what to improve!

Or do you mean you'll merge it?

[domenkozar]

Thanks! Pushed and updated in 8408136

[matthiasbeyer]

Why not merging it via Merge?

[edolstra]

Title should be capitalized.