add author's note to episub

pubsub/gossipsub/episub.md

@@ -5,6 +5,13 @@ Revision: draft 1, 2018-06-28
 Authors:
 - vyzo (vyzo@hackzen.org)
 
+Author's note:
+- This is based on an earlier research draft about an epidemic broadcast protocol
+  for libp2p pubsub.
+  It serves as reference for the design of episub, an extended gossipsub router
+  optimized for single source multicast and scenarios with a few fixed sources
+  broadcasting to a large number of clients in a topic.
+
 <!-- toc -->
 
 - [Introduction](#introduction)