Update README.md

Author: cloudwebrtc

README.md

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 # dart-sip-ua
+ [![slack](https://img.shields.io/badge/join-us%20on%20slack-gray.svg?longCache=true&logo=slack&colorB=brightgreen)](https://join.slack.com/t/flutterwebrtc/shared_invite/zt-q83o7y1s-FExGLWEvtkPKM8ku_F8cEQ)
+ 
 A dart-lang version of the SIP UA stack, ported from [JsSIP](https://github.com/versatica/JsSIP).
 
 ## Overview