[docs] Add note about compatibility (#238)

Add note about compatibility, as described in https://github.com/socketio/socket.io-redis/releases/tag/5.0.0

Author: luislobo

History.md

@@ -17,6 +17,8 @@
   * [perf] Use notepack instead of msgpack-lite (#218)
   * [perf] Use pattern matching at the namespace level (#217)
 
+Note: This is the first version compatible with socket.io v2. Please use previous versions (4.x) for socket.io v1.x.
+
 4.0.1 / 2017-05-11
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