Add Nix package to README?

[thibmaek]

Not sure if you're open to adopt install/usage instructions for non-official (maintained by others) install methods?
I saw macmon is also available on nixpkgs and I primarily use darwin-nix on my system to manage tools and installed it like that.

https://search.nixos.org/packages?channel=24.11&show=macmon&from=0&size=50&sort=relevance&type=packages&query=macmon

Let me know, I'd gladly create a PR