Firestore/CHANGELOG.md: Add change log entry (NOTE: update with actual PR number)

Author: dconeybe

Firestore/CHANGELOG.md

@@ -1,3 +1,9 @@
+# Unreleased
+- [feature] Implemented an optimization in the local cache synchronization logic
+  that reduces the number of billed document reads when documents were deleted
+  on the server while the client was not actively listening to the query
+  (e.g. while the client was offline). (#INSERT_PR_NUMBER_HERE)
+
 # 10.11.0
 - [feature] Expose MultiDb API for public preview. (#10465)
 - [fixed] Fixed a compilation warning related to integer casting. (#11332)