adds a root changelog (firebase#4009)

* adds a root changelog

* Update CHANGELOG.md

Co-authored-by: Feiyang <[email protected]>

Author: dimitropoulos

CHANGELOG.md

@@ -0,0 +1,5 @@
+# Changelog
+
+You can view the changelog for the packages within this monorepo in `packages/<package name>/CHANGELOG.md`. For example, you will find the changelog for the Firestore package at [`packages/firestore/CHANGELOG.md`](./packages/firestore/CHANGELOG.md).
+
+Additionally, you can view our official release notes for the entire monorepo at the [firebase support page](https://firebase.google.com/support/release-notes/js).