[chat] add gitter badge

Author: hshoff

README.md

@@ -1551,6 +1551,8 @@
 
 ## Chat With Us About JavaScript
 
+  [![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/airbnb/javascript?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
+
   - Find us on [gitter](https://gitter.im/airbnb/javascript).
 
 ## Contributors