Add a Gitter chat badge to README.md

### gittertestbot/readme-dot-markdown now has a Chat Room on Gitter

@gittertestbot has just created a chat room. You can visit it here: [https://gitter.im/gittertestbot/readme-dot-markdown](https://gitter.im/gittertestbot/readme-dot-markdown?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&content=body_link).

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

[![Gitter](http://localhost:4000/Join Chat.svg)](http://localhost:5000/gittertestbot/readme-dot-markdown?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=body_badge)

Happy chatting.