Add a changelog entry for "Stop persisting last acknowledged batch ID" (#2253)

Author: var-const

Firestore/CHANGELOG.md

@@ -8,6 +8,9 @@
   exist (instead of returning a nil DocumentSnapshot). Code that was previously
   doing `if (snapshot) { ... }` must be changed to
   `if (snapshot.exists) { ... }`.
+- [fixed] Fixed a crash that could happen when the app is shut down after
+  a write has been sent to the server but before it has been received on
+  a listener (#2237).
 
 # v0.16.1
 - [fixed] Offline persistence now properly records schema downgrades. This is a