Add Gitter badge (#2179)

README.md

@@ -1,6 +1,8 @@
 vis.js
 ==================
 
+[![Join the chat at https://gitter.im/vis-js/Lobby](https://badges.gitter.im/vis-js/Lobby.svg)](https://gitter.im/vis-js/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 <a href="https://github.com/almende/vis/issues/1781" target="_blank">
   <img align="right" src="https://raw.githubusercontent.com/almende/vis/master/misc/we_need_help.png">
 </a>