Update README.md

Related: #447

README.md

@@ -138,7 +138,8 @@ Reload .vimrc and `:PlugInstall` to install plugins.
 | `g:plug_retries`    | 2                                 | Number of retries in case of timeout (*Ruby & Python*) |
 | `g:plug_shallow`    | 1                                 | Use shallow clone                                      |
 | `g:plug_window`     | `vertical topleft new`            | Command to open plug window                            |
-| `g:plug_url_format` | `https://git::@github.com/%s.git` | `printf` format to build repo URL                      |
+| `g:plug_url_format` | `https://git::@github.com/%s.git` | `printf` format to build repo URL (Only applies to the subsequent `Plug` commands) |
+
 
 ### Keybindings