Missing documentation on how repositories become official

[friedow]

@zimbatm and me just talked about how github repositories could advance from the nix-community to the NixOS github organization. Currently there is no documentation on this in the orga repository except from this sentence in /doc/resources.md:

Currently there is no established process for how to make new official resources or make existing resources official.

However, @zimbatm also mentioned that the current process to make a new official repo would be to create an issue in the orga repository and go from there. This should be reflected in /doc/resources.md.