add a readme.md file to explain the ping/pong messages

Signed-off-by: Leo Chen <leo@harmony.one>

api/proto/discovery/readme.md

@@ -0,0 +1,27 @@
+## Ping
+
+Ping message sent by new node who wants to join in the consensus.
+The message is broadcasted to the shard.
+It contains the public BLS key of consensus and the peerID of the node.
+For backward compatibility, it still contains the IP/Port of the node,
+but it will be removed once the full integration of libp2p is finished as the IP/Port is not needed.
+
+It also contains a Role field to indicate if the node is a client node or regular node, as client node
+won't join the consensus.
+
+## Pong
+
+Pong message is sent by leader to all validators, once the leader has enough validators.
+It contains a list of peers and the corresponding BLS public keys.
+Noted, the list of peers may not be needed once we have libp2p fully integrated.
+The order of the peers and keys are important to the consensus.
+
+At bootstrap, the Pong message is sent out and then the consensus should start.
+
+## TODO
+
+The following two todo should be worked on once we have full libp2p integration.
+For network security reason, we should in general not expose the IP/Port of the node.
+
+-[] remove peer info in Ping message, only keep peerID, which should be sufficient for p2p communication.
+-[] remove peer list from Pong message.