update readme

Author: duytq94

README.md

@@ -6,6 +6,11 @@
 * Watch the demo video for more clarity: https://youtu.be/MOHE68LI5Eg
 * Check my general guidance to know more and how to do it: https://medium.com/@duytq94/building-a-chat-app-with-flutter-and-firebase-from-scratch-9eaa7f41782e
 
+## Extension
+* Push notification messages.
+* Demo: https://youtu.be/TL--RKbNPt8
+* Article: https://medium.com/@duytq94/flutter-chat-app-extended-push-notification-messages-a26c669f4675
+
 ## Screenshot
 
-<img src="https://raw.githubusercontent.com/duytq94/flutter-chat-demo/master/screenshots/FlutterChatDemo.gif" height="50%" width="50%">
+<img src="https://raw.githubusercontent.com/duytq94/flutter-chat-demo/master/screenshots/FlutterChatDemo.gif" height="50%" width="50%">