update release notes

Signed-off-by: Rehan Durrani <rehan@ponder.io>

Author: Rehan Durrani

docs/release_notes/release_notes-0.16.0.rst

@@ -20,7 +20,7 @@ Key Features and Updates
 * XGBoost enhancements
   *
 * Developer API enhancements
-  *
+  * FEAT-#4574: Warn users when pre-initialized Ray cluster is not using all available memory (#4575)
 * Update testing suite
   * TEST-#4508: Reduce test_partition_api pytest threads to deflake it (#4551)
   * TEST-#4550: Use much less data in test_partition_api (#4554)
@@ -34,3 +34,4 @@ Contributors
 ------------
 @mvashishtha
 @prutskov
+@RehanSD