Update messaging notes

docs/messaging.txt

+
 Messaging notes
 ===============
 
-To replicate messages between multiple SIP devices, discover PGP keys and
-receive messages while offline, one must install SylkServer and configure it
-with Cassandra storage backend, see https://sylkserver.com
+Blink support two types of media for messaging.
+
+1. Session based chat using MSRP protocol (real-time chat)
+2. SIP Message method (asynchronous messaging)
+
+
+Real-time chat
+==============
+
+To start a session based chat open the Chat window, click on the session
+menu button and select Start Real Time Chat option or right click a contact
+and select the same option.  Once established, the session using MSRP media
+uses TLS transport and end-to-end encryption using OTR is by default
+enabled.
 
-These features are compatible with other SIP clients that implement the same
-API, currently Blink for Mac and Sylk Mobile are known to support the same
-API as Blink Qt.
 
-The SIP Proxy for the account must fork a copy of all SIP messages to
-SylkServer.  When properly configured, SylkServer will respond to SIP
-messages sent by Blink with the following content types:
+Asynchronous messaging
+======================
+
+To send messages asynchronously, which may be delivered either in real time
+of at a later time, just type the message to the selected contact.  These
+messages will be delivered using SIP Message method.  By default, encryption
+using PGP mechanism is used, if there is a public key present for the SIP
+address of the recipient, the messages will be automatically encrypted using
+it.  As an alternative, one can start an end-to-end OTR encrypted session
+between two end-points, in this case PGP encryption will be disabled and
+messages will only be delivered to the end-point that accepted the OTR
+session.
+
+To replicate messages between multiple SIP devices, discover PGP public keys
+and receive messages while offline, one must install SylkServer and
+configure it with Cassandra storage backend.
+
+Blink asynchronous messaging features are compatible with other SIP clients
+that implement Sylk Server messaging API.  Sylk Desktop and Sylk Mobile have
+been tested and are known to support the same messaging API as Blink.  All
+messages exchanged with one SIP client are replicated to all other SIP
+clients configured with the same SIP account.
+
+The SIP Proxy must fork a copy of all SIP messages to SylkServer.  When
+properly configured, SylkServer will respond to SIP messages sent by Blink
+with the following content types:
 
   * application/sylk-api-token (will auto discover the URL and index for offline storage)
   * application/sylk-api-pgp-key-lookup (will return the OpenPGP key if exists)
@@ -32,4 +64,3 @@ The public keys of the recipients are looked up in SylkServer at the start
 of a messaging session.  If a key is found, it will be returned as SIP
 message with a special content type recognized by Blink and saved in a file
 e.g. ~/.blink/keys/user@domain.pubkey
-