vim-plugins: document `nix-shell -I` usage for `vim-plugin-names-to-nix` usage

I use my own fork for NixOS development/testing, but `nixos-unstable`
for my system configuration.

Whenever I add a new plugin and I regenerate the expressions I need to
change the `NIX_PATH` variable to my own fork. As this is something I
tend to forget I think it's better to document this.