Create FCM's CHANGELOG.md with a release note.

This RN is meant for the release scheduled on 2020-04-02.
The fix will addresses the issues in #2712

Author: Kai Wu

packages/messaging/CHANGELOG.md

@@ -0,0 +1,4 @@
+# Unreleased
+- [fixed] Fixed an issue introduced in 7.7.0 where the FCM switched to provide base64-encoded VAPID key
+to [PushManager](https://developer.mozilla.org/en-US/docs/Web/API/PushManager) for push 
+subscribtion. For backward compatibility, the SDK switched back to use VAPID key in type (ArrayBuffer)[https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer].