Add CHANGELOG for latency-compensated query fix (#622)

* Add CHANGELOG for latency-compensated query fix

* made 20.2.0

* add issue ref

Author: Hui-Wu

firebase-firestore/CHANGELOG.md

@@ -1,6 +1,9 @@
 # Unreleased
 
 # 20.2.0
+- [fixed] Fixed an issue where query results were temporarily missing documents
+  that previously had not matched but had been updated to now match the
+  query (#155).
 - [feature] Added a `@DocumentId` annotation which can be used on a
   `DocumentReference` or `String` property in a POJO to indicate that the SDK
   should automatically populate it with the document's ID.