Add a link to the release notes in README.md (#842)

Every time there is a new release, I need to go searching for this - not sure that everyone is aware of where these are placed online, seems to make sense to have a link included here.

Author: mwaldstein

README.md

@@ -15,6 +15,8 @@ applications using Firebase services. This SDK is distributed via:
 To get started using Firebase, see
 [Add Firebase to your JavaScript Project](https://firebase.google.com/docs/web/setup).
 
+Current [Release Notes](https://firebase.google.com/support/release-notes/js)
+
 ## SDK Dev Workflow
 
 ### Prerequisites