Add a Gitter chat badge to README.txt

### yosuke/rtshell now has a Chat Room on Gitter

@yosuke has just created a chat room. You can visit it here: https://gitter.im/yosuke/rtshell.

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

[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/yosuke/rtshell?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)

Happy chatting.