Advise on installation of missing clutter package in case of broken s…

…ettings window

README.md

@@ -25,6 +25,10 @@ An extension for Gnome-Shell to show Persian date and calendar
    * If you don't have `gnome-tweaks`, first install it. Then open it and go to the “extensions” section. There is a button in the title bar of the window, make sure it's enabled.
    * If you cannot see the “extensions” section in the `gnome-tweaks` application, congratulation, you are using a more up-to-date version of Gnome, so in this case, please use the `gnome-extensions` application and make sure the button is enabled.
 * If none of the above works, you can install [Extension Manager](https://github.com/mjakeman/extension-manager#-installing), then install and activate the Persian Calendar extension through it. Extension Manager is a tool for managing GNOME Shell Extensions.
+* The extension seems to work, but the settings window in extension manager reports the following message "Error: Requiring Clutter, version none: Typelib file for namespace '`Clutter'"
+   * This error is a result of some clutter libraries no longer being bundled with your OS if you use Ubuntu, please open a terminal and execute 
+   `$sudo apt install -y gir1.2-gtkclutter-1.0`
+   * Or equivalent command for your distribution, and restart your windows manager or log out and log back in.
 
 ### Date formatting:
 | Format Code | Meaning | Example Output |