update readme

Author: ianlin

README.md

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 # React Native VoIP Push Notification
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+[![npm version](https://badge.fury.io/js/react-native-voip-push-notification.svg)](https://badge.fury.io/js/react-native-voip-push-notification)
+[![npm downloads](https://img.shields.io/npm/dm/react-native-voip-push-notification.svg?maxAge=2592000)](https://img.shields.io/npm/dm/react-native-voip-push-notification.svg?maxAge=2592000)
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 React Native VoIP Push Notification - Currently iOS >= 8.0 only
 
 ## Motivation