Add CHANGELOG.md entry for reverting the usage of 'crypto' (#2873)

Author: dconeybe

packages/firestore/CHANGELOG.md

@@ -1,4 +1,6 @@
 # Unreleased
+- [fixed] Temporarily reverted the use of window.crypto to generate document
+  IDs to address compatibility issues with IE 11, WebWorkers, and React Native.
 - [changed] Firestore now limits the number of concurrent document lookups it
   will perform when resolving inconsistencies in the local cache (#2683).
 - [changed] Changed the in-memory representation of Firestore documents to