Update Group chat feature (design draft)

Group-chat-feature-(design-draft).md

@@ -11,6 +11,8 @@ So, the *swarm* is defined by:
 4. Non-repudiation. Devices must be able to verify old messages validity and to replay the whole history.
 5. PFS on the transport. Storage is managed by the device.
 
+Main idea is to get a synchronized merkle tree with the participants.
+
 # Scenarios
 
 ## Create a Swarm