add HEADs message format

rfc/0004_ws2p_v1.md

@@ -225,6 +225,19 @@ The set of heads known by a duniter node at the moment is public information ava
 
 Each new head received from the same node overwrites the previous head in memory, so that only the current state of the network is stored.
 
+HEADs message format :
+
+    {
+        name: "HEAD",
+        body: {
+            heads: [
+                HEAD_0,
+                HEAD_1,
+                ...
+            ]
+        }
+    }
+
 ### HEAD v0
 
 Accepted since duniter-ts **v1.6.0**