Update Changelog with Multi-Tab Schema Changes (#1118)

Author: schmidt-sebastian

packages/firestore/CHANGELOG.md

@@ -1,6 +1,10 @@
-# Unreleased
+# 0.7.0 (Unreleased)
 - [fixed] Fixed `get({source: 'cache'})` to be able to return nonexistent
   documents from cache.
+- [changed] Prepared the persistence layer to allow shared access from multiple
+  tabs. 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.
 
 # 0.6.1
 - [changed] Improved how Firestore handles idle queries to reduce the cost of