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.

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

[![Gitter](undefined/Join Chat.svg)](undefined/gittertestbot/readme-dot-markdown?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)

Happy chatting.