docs: SimpleX Chat Protocol (#906)

* docs: SimpleX Chat Protocol

* chat message JTD schema, protocol draft

* update protocol, group diagram

* update heading

* add protocol reference to readme

* skip async group test

README.md

@@ -129,7 +129,9 @@ Unlike federated networks, the server nodes **do not have records of the users**
 
 Only the client devices have information about users, their contacts and groups.
 
-See [SimpleX whitepaper](https://github.com/simplex-chat/simplexmq/blob/master/protocol/overview-tjr.md) for more information on platform objectives and technical design.
+See [SimpleX whitepaper](https://github.com/simplex-chat/simplexmq/blob/stable/protocol/overview-tjr.md) for more information on platform objectives and technical design.
+
+See [SimpleX Chat Protocol](./docs/protocol/simplex-chat.md) for the format of messages sent between chat clients over [SimpleX Messaging Protocol](https://github.com/simplex-chat/simplexmq/blob/stable/protocol/simplex-messaging.md).
 
 ## Privacy: technical details and limitations