Add changelog entry

Author: madsmtm

objc2/CHANGELOG.md

@@ -6,6 +6,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased - YYYY-MM-DD
 
+### Added
+* Added `msg_send_id!` to help with following Objective-C's memory management
+  rules. **It is highly recommended that you use this instead of doing memory
+  management yourself!**
+
 
 ## 0.3.0-beta.0 - 2022-06-13