Update CHANGELOG.md

Author: rlazo

firebase-perf/CHANGELOG.md

@@ -1,4 +1,7 @@
 # Unreleased
+* [fixed] Fixed a bug where screen traces were not capturing frame metrics for multi-activity apps.
+* [feature] Added support for measuring screen performance metrics for "Activity Fragments" out-of-the-box.
+* [fixed] Excluded custom attributes whose key/value length is 0.
 
 # 20.2.0
 * [unchanged] Updated to accommodate the release of the updated