Add NixOS install instructions

[leifhelm]

Hi, I package cfonts for nix.
This PR add one of the options of installing it on an NixOS system.
Creating a shell with cfonts on $PATH can be done like this

nix-shell -p cfonts

Simply running it could be done with:

nix run nixpkgs#cfonts

If the experiential features flakes and nix-command are enabled.

[dominikwilkowski]

Thank you for this.
What needs to happen when an update it pushed for cfonts? Will this nix pick up the update automatically? Is there a place for the recipe to be maintained? Like AUR has it's own repo)
Also I assume you're using the rust version yes?

[leifhelm]

What needs to happen when an update it pushed for cfonts?

The version and the hash have to be updated here. I typically update the version, build it and update any mismatching hashes. A typical PR looks like this

Will this nix pick up the update automatically?

No. When I get the notification of a release I update it manually.

Also I assume you're using the rust version yes?

Yes it is the rust version.

[dominikwilkowski]

This is great. Thank you
Last request: Can you move it between Fedora and MacPorts?
Feels more like a distribution level thing.