Request: Add install instructions to README.md

Hey there, This looks great, but I'm pretty new to Nix and have been wrapping my head around it for a few weeks now. Would it be possible to get some installation instructions added to the readme? FWIW I'm on a flake-based NixOS config. I see the flake.nix in this repo adds the package to nixpkgs' overlays, but I'm not sure how to best add that to my flake, as I suppose just adding to the inputs won't be sufficient. Thanks so much **Edit:** Hey, I just found out I can run `nix run github:fzakaria/nix-auto-follow` and it just works -- magic! **Edit 2:** And I've just seen that the `nix run` command is _in the README_. Apologies; I was tired/am a fool.