Add a Gitter chat badge to README.md

### gitterhq/sandbox now has a Chat Room on Gitter

@_testuser_2291411737321806 has just created a chat room. You can visit it here: https://gitter.im/gitterhq/sandbox.

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

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

Happy chatting.