define Data message type

README.md

@@ -89,12 +89,21 @@ It has no payload.
 
 A message which MAY be originated by a node to check for reachability of other nodes in the network.
 Nodes which receive this message SHOULD respond with a *Pong* message.
+It has no payload.
 
 #### Message Type *Pong*
 
 - Message Type `0x04`
 
 A message which MUST ONLY be originated by a node in response to a previous *Ping* message.
+It has no payload.
+
+#### Message Type *Data*
+
+- Message Type `0x10`
+
+This message is used for actual data transfer between nodes.
+It's payload makes up the remainder of the UDP datagram and the content is up to the upper protocol to decide.
 
 
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