Add a zsh completion script
Add a zsh completion script

Based on @clhodapp's suggestion in https://github.com/spwhitt/nix-zsh-completions/issues/32#issuecomment-705315356 and adapted to use the description of the completions

rycee/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix