Add installation instructions for Nix

I found myself using hashi-up in a Nix shell environment so I decided to contribute it to Nixpkgs: https://github.com/NixOS/nixpkgs/pull/194288. The PR is under review but should land soon-ish. You're under no responsibility to add Nix installation docs, of course, but I wanted to make you aware that this is now an option for users ❤️