Add a Gitter chat badge to README.md

### tomaka/simple-gl-rs now has a Chat Room on Gitter

@tomaka has just created a chat room. You can visit it here: https://gitter.im/tomaka/simple-gl-rs.

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

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

Happy chatting.