Add changelog entry for CG query internals. (#1588)

Author: mikelehen

packages/firestore/CHANGELOG.md

@@ -4,6 +4,10 @@
   values safely without transactions.
 - [changed] Improved performance when querying over documents that contain
   subcollections.
+- [changed] Prepared the persistence layer to support collection group queries.
+  While this feature is not yet available, all schema changes are included
+  in this release. Once you upgrade, you will not be able to use an older version
+  of the Firestore SDK with persistence enabled.
 
 # 1.0.4
 - [fixed] Fixed an uncaught promise error occurring when `enablePersistence()`