Add module docstring to serialization.py

lib/tfw/networking/serialization.py

@@ -1,5 +1,25 @@
 # Copyright (C) 2018 Avatao.com Innovative Learning Kft.
 # All Rights Reserved. See LICENSE file for details.
+"""
+TFW JSON message format
+
+message:
+{
+    "key":     string,    # addressing
+    "data":    {...},     # payload
+    "trigger": string     # FSM trigger
+}
+
+ZeroMQ's sub-pub sockets use enveloped messages
+(http://zguide.zeromq.org/page:all#Pub-Sub-Message-Envelopes)
+and TFW also uses them internally. This means that on ZMQ sockets
+we always send the messages key separatley and then the actual
+message (which contains the key as well) like so:
+
+socket.send_multipart([message['key'], message])
+
+The purpose of this module is abstracting away this low level behaviour.
+"""
 
 import json