Add changelog entry for communication rewrite

Add entry to changelog detailing the rewrite of the communication layer.

CHANGELOG.md

@@ -16,6 +16,9 @@ Version 2.0.0
   instances, starting and stopping them individually.  To manage this the
   local command line interface has been replaced with a remote interface on
   the master server that can be accessed through the clusterctl cli tool.
+- Rewritten the communication between slaves and the master from scratch.  The
+  new system is based on a WebSocket connection between the slaves and the
+  master server and provides efficient validated bi-directonal communication.
 
 ### Changes