Improve and document dependency injection API

[DavHau]

see: https://github.com/nix-community/dream2nix/issues/117#issuecomment-1107676009

[DavHau]

Documentation is already done via example flake in readme

[happysalada]

This is looking really good! One point that I would add, is instead of using foo, I would add another example with the @ sign (because those appear as at in nix cli, it's not immediately obvious. Let me add something and you can tell me what you think.