Add a Gitter chat badge to README.md

### malditogeek/vmux now has a Chat Room on Gitter

@gittertestbot has just created a chat room. You can visit it here: https://gitter.im/malditogeek/vmux.

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

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

Happy chatting.