Add note about installing on NixOS

[marcospb19]

ouch was added to nixpkgs in NixOS/nixpkgs#131538, but we don't have any instruction or citation to that in the README.

Bumping @figsoda, can you solve this?

I'm not experienced with NixOS, so I don't even know how it's installed. Maybe you could add the command to install, or just the link to the package.

[figsoda]

There is way too many ways to install a package with nix(os), I would probably put a repology badge like this

There is a couple more if you want something else: https://repology.org/project/ouch/badges

[marcospb19]

Oh, it's the same as the one used in https://github.com/sharkdp/bat#installation.

Yeah, I think it's great!