Add documentation link

Author: FreePhoenix888

README.md

@@ -55,4 +55,6 @@ To receive messages on Client read [Firebase Cloud Messaging Documentation](http
 
 # Library
 ## Library Usage
-See [Documentation] for examples and API
+See [Documentation] for examples and API
+
+[Documentation]: https://deep-foundation.github.io/firebase-push-notification/