Add NixOS module to installation instructions

Note that I'm unsure about the description. "by NixOS Contributors" is somewhat generic, but it's impractical to point to a single person as a lot of contributors have contributed changes to GitLab module, https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/services/misc/gitlab.nix shows 35 contributors (and there are more files, so there are possibly more contributors). The package has four listed current maintainers (Franz Pletz, Robin Gloster, Kristoffer Thømt Ravneberg, Kim Lindberger).

Why is this change being made?

Making the list of community contributed packages more complete.

Does this MR meet the acceptance criteria?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what