Create FCM's CHANGELOG.md with a release note. (#2837)

* 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

* Update CHANGELOG.md

Improves wording on Eric's feedback. Thansk Eric

* Update CHANGELOG.md

Fix a type

Co-authored-by: Kai Wu <[email protected]>

Author: zwu52

packages/messaging/CHANGELOG.md

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