Add installation instructions for NixOS
Add installation instructions for NixOS

The package set `nixpkgs`, NixOS relies on provides `uutils-coreutils`
out of the box since 18.03/unstable.

See https://github.com/NixOS/nixpkgs/pull/34840

Arcterus/coreutils,Arcterus/coreutils,tempbottle/coreutils,tempbottle/coreutils,uutils/coreutils,uutils/coreutils,uutils/coreutils