update readme

Author: ianlin

README.md

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 # react-native-incall-manager
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+[![npm version](https://badge.fury.io/js/react-native-incall-manager.svg)](https://badge.fury.io/js/react-native-incall-manager)
+[![npm downloads](https://img.shields.io/npm/dm/react-native-incall-manager.svg?maxAge=2592000)](https://img.shields.io/npm/dm/react-native-incall-manager.svg?maxAge=2592000)
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 Handling media-routes/sensors/events during a audio/video chat on React Native
 
 ## Purpose: