Add Gitter badge (#2179)

README.md

@@ -1,5 +1,6 @@
-vis.js
-==================
+# 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/blob/develop/misc/we_need_help.md">
   <img align="right" src="https://raw.githubusercontent.com/almende/vis/master/misc/we_need_help.png">