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](http://localhost:4000/Join Chat.svg)](http://localhost:5000/trevorah/usernique?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)

Happy chatting.