replaced broken herokuapp links with link to google group

Author: cjac

README.md

@@ -1,5 +1,5 @@
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
-[![Join CDAP community](https://cdap-users.herokuapp.com/badge.svg?t=salesforce-plugins)](https://cdap-users.herokuapp.com?t=1)
+[![Join CDAP community](https://groups.google.com/g/cdap-user)
 
 # Overview