Add et_Chat callback documentation

callbacks.rst

@@ -231,3 +231,13 @@ et_SpawnEntitiesFromString()
 ----------------------------
 
 Called when an entity definition is parsed to spawn gentities.
+
+
+et_Chat( sender, receiver, message )
+---------------------------------------------
+
+Called whenever a player sends a chat message. It is called for each player receiving the message.
+
+* **sender** is the player sending the message.
+* **receiver** is the player receiving the message.
+* **message** is the chat message.