Update README.md

Author: kevzlou7979

README.md

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 GwtMaterialDesign plus [AmCharts](https://www.amcharts.com/) Version 4 provides you professional and nice looking charts.
 
 
-[![Build Status](https://travis-ci.org/GwtMaterialDesign/gwt-material-amcharts.svg?branch=master)](https://travis-ci.org/GwtMaterialDesign/gwt-material-amcharts4) [![Join the chat at https://gitter.im/GwtMaterialDesign/gwt-material](https://img.shields.io/badge/GITTER-join%20chat-green.svg)](https://gitter.im/GwtMaterialDesign/gwt-material?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Build Status](https://travis-ci.org/GwtMaterialDesign/gwt-material-amcharts.svg?branch=master)](https://travis-ci.org/GwtMaterialDesign/gwt-material-amcharts4) [![Join us](https://img.shields.io/badge/slack-channel-purple)](https://gmd-project.slack.com)
 
 ## Demo
 * https://gwtmaterialdesign.github.io/gmd-amcharts4-demo/