Improve docs for using it with home-manager as a NixOS module

When configuring something that requires the usage of cosmicLib, I wasn't sure how to import it. I tried lib.cosmicLib, inputs.cosmic-manager.lib and a few others with no luck.

What worked for me is cosmicManager = config.home-manager.users.cosmic-user.lib. It's also confusing because AFAIU, it should be called something like homeManagerLib (or just lib) and not cosmicLib.