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.

pkgs/misc/vim-plugins/default.nix

@@ -16,7 +16,7 @@ in
 
 # TL;DR
 # Add your plugin to ./vim-plugin-names
-# Regenerate via `nix-shell -p vimPlugins.pluginnames2nix --command "vim-plugin-names-to-nix"`
+# Regenerate via `nix-shell -I nixpkgs=/path/to/your/local/fork -p vimPlugins.pluginnames2nix --command "vim-plugin-names-to-nix"`
 # Copy the generated expression(s) into this file.
 # If plugin is complicated then make changes to ./vim2nix/additional-nix-code