Add license badge

Author: dheid

README.md

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 # JavaPNS
 
 ![Maven Verify](https://github.com/drjekyll-org/javapns/workflows/Maven%20Verify/badge.svg)
+![GitHub](https://img.shields.io/github/license/drjekyll-org/javapns)
 
 JavaPNS is a Java library to send notifications through the Apple Push Notification Service (APNS). It
 allows to push notifications to iOS devices through APNS.