Add a Gitter chat badge to README.textile

### suprememoocow/Ext.ux.Exporter now has a Chat Room on Gitter

@gittertestbot has just created a chat room. You can visit it here: https://gitter.im/suprememoocow/Ext.ux.Exporter.

This pull-request adds this badge to your README.textile:

[![Gitter](http://localhost:4000/Join Chat.svg)](http://localhost:5000/suprememoocow/Ext.ux.Exporter?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)

Happy chatting.