release notes (#2508)

* release notes

* mention connection-id tracking

Author: mgravell

docs/ReleaseNotes.md

@@ -8,7 +8,8 @@ Current package versions:
 
 ## Unreleased
 
-(none)
+- Fix [#2507](https://github.com/StackExchange/StackExchange.Redis/issues/2507): Pub/sub with multi-item payloads should be usable ([#2508 by mgravell](https://github.com/StackExchange/StackExchange.Redis/pull/2508))
+- Add: connection-id tracking (internal only, no public API) ([#2508 by mgravell](https://github.com/StackExchange/StackExchange.Redis/pull/2508))
 
 ## 2.6.122