Add a Gitter chat badge to README.md

### trevorah/usernique now has a Chat Room on Gitter

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

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

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

Happy chatting.