add gitter chat room

Author: gutenye

README.md

@@ -1,6 +1,9 @@
 polymer-sortablejs
 -------------------
 
+[![Gitter][gitter-image]][gitter-url]
+
+
 A Polymer binding to [SortableJS](https://github.com/RubaXa/Sortable/).
 
 Demo: http://rubaxa.github.io/Sortable/
@@ -23,3 +26,6 @@ Demo: http://rubaxa.github.io/Sortable/
 ```
 $ bower install polymer-sortablejs
 ```
+
+[gitter-image]: http://img.shields.io/badge/+%20GITTER-JOIN%20CHAT%20%E2%86%92-1DCE73.svg?style=flat-square
+[gitter-url]: https://gitter.im/sortable-js/polymer-sortablejs