Add Gitter badge

Author: gitter-badger

README.md

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 <img src="http://i.imgur.com/k7iZMbE.png" />
 # gwt-material-addins [![Build Status](https://travis-ci.org/GwtMaterialDesign/gwt-material-addins.svg?branch=master)](https://travis-ci.org/GwtMaterialDesign/gwt-material-addins)
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+[![Join the chat at https://gitter.im/GwtMaterialDesign/gwt-material-addins](https://badges.gitter.im/GwtMaterialDesign/gwt-material-addins.svg)](https://gitter.im/GwtMaterialDesign/gwt-material-addins?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 Gwt Material Design Extra Components for https://github.com/GwtMaterialDesign/gwt-material <br>
 
 ## Current Version