pluginupdate.py: add support for adding/updating individual plugins

doc/languages-frameworks/vim.section.md

@@ -264,10 +264,15 @@ nix-shell -p vimPluginsUpdater --run 'vim-plugins-updater --github-token=mytoken
 Alternatively, set the number of processes to a lower count to avoid rate-limiting.
 
 ```sh
-
 nix-shell -p vimPluginsUpdater --run 'vim-plugins-updater --proc 1'
 ```
 
+If you want to update only certain plugins, you can specify them after the `update` command. Note that you must use the same plugin names as the `pkgs/applications/editors/vim/plugins/vim-plugin-names` file.
+
+```sh
+nix-shell -p vimPluginsUpdater --run 'vim-plugins-updater update "nvim-treesitter" "LazyVim"'
+```
+
 ## How to maintain an out-of-tree overlay of vim plugins ? {#vim-out-of-tree-overlays}
 
 You can use the updater script to generate basic packages out of a custom vim