Added notes about usage of messaging features

INSTALL

@@ -94,3 +94,32 @@ fi
 ./build_inplace
 ./run
 
+
+Notes
+=====
+
+Messaging
+
+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
+
+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.
+
+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:
+
+  * 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)
+
+All settings are subsequently auto-discovered by Blink.
+
+To replicate messages from multiple devices they all must use the same
+private PGP key.  The import PGP panel is presented at start of messaging
+sessions for each account.  The key can be exported from another device that
+implement the same API or can be copied manually to .blink configuration
+folder.
+ 
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