Update persistent_notification.markdown (#619)

included a little 'gotcha' that caught me out for a bit. =)

source/_components/persistent_notification.markdown

@@ -52,3 +52,5 @@ Choose <img src='/images/screenshots/developer-tool-services-icon.png' alt='serv
 }
 ```
 This will create the notification entry shown above.
+
+NOTE: if you have defined a ```default_view:``` in your ```Groups:``` configuration you will need to include ```persistent_notification.<notification_id>``` e.g. ```persistent_notification.1234``` as per the example above, to your Groups configuration, in order to see the notification after creating it.