README.md: Exclude 'Ubuntu' gnome-shell session from our supported de…

…sktops

Their sessions override user-themes in many places.
It's around time to say good-bye to Ubuntu. It means I'm OK to pin Ubuntu
as an unsupported distribution now.

README.md

@@ -93,7 +93,7 @@ Supported Desktop Environments
 Unsupported Gtk+ Based Desktop(s)
 -------------------------------
  * Pantheon
- * Unity7
+ * Unity7 and Ubuntu customed Gnome-Shell
 
  > **Note:**
  >