Update CHANGELOG.md (#694)

Author: laurenzlong

CHANGELOG.md

@@ -1 +1,3 @@
-- Pin `@types/express` version to 4.17.3 to fix type definition issue (#685)
+- Pin `@types/express` version to 4.17.3 to fix type definition issue (Issue #685).
+- Firestore onCreate, onUpdate, and onDelete now receive a `QueryDocumentSnapshot` instead of `DocumentSnapshot`, which guarantees that data is not undefined (Issue #659).
+- Modify return type of `DataSnapshot.forEach` to `boolean | void` match `firebase-admin` SDK.