update CHANGELOG

Author: EmandM

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -10,6 +10,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Added
 
+- Added mappings between `ClientId` and `TransportId`. (#3515)
 - Added `SinglePlayerTransport` that provides the ability to start as a host for a single player network session. (#3475)
 - When using UnityTransport >=2.4 and Unity >= 6000.1.0a1, SetConnectionData will accept a fully qualified hostname instead of an IP as a connect address on the client side. (#3440)