Add a release note for the fix to #1548 (#1024)

Author: wilhuff

packages/firestore/CHANGELOG.md

@@ -1,4 +1,7 @@
 # Unreleased
+- [fixed] Fixed an issue where documents could be cached as deleted when
+  they should not have been, leading to queries returning fewer results than
+  they should (firebase/firebase-ios-sdk#1548).
 - [feature] Added `firebase.firestore.FieldValue.arrayUnion()` and
   `firebase.firestore.FieldValue.arrayRemove()` to atomically add and remove
   elements from an array field in a document.