Update README.md

Author: rwieruch

README.md

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 # react-redux-firebase-authentication
 
+[![Slack](https://slack-the-road-to-learn-react.wieruch.com/badge.svg)](https://slack-the-road-to-learn-react.wieruch.com/)
+
 * Found in [Taming the State in React](https://roadtoreact.com/course-details?courseId=TAMING_THE_STATE)
 * [Live](https://react-firebase-authentication.wieruch.com/)
 * [Tutorial](https://www.robinwieruch.de/complete-firebase-authentication-react-tutorial/)