update readme

Author: Wahid Nasri

README.md

@@ -17,7 +17,7 @@ The Broker should install [HiveMQ Chat-Extension](https://github.com/WahidNasri/
 - [x] Persist User, Messages and rooms using local database
 - [x] ChatMarker (In Progress)
 - [x] Invite a user to chat
-- [ ] Group Chat
+- [x] Group Chat
 - [ ] Presence (Done on backend side)
 - [x] File Previews (PDF and images Done)
 - [ ] Push Notifications When App is in Background.