Add a Gitter chat badge to README.md

### WordPress/book now has a Chat Room on Gitter

@smckeown has just created a chat room. You can visit it here: [https://gitter.im/WordPress/book](https://gitter.im/WordPress/book?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&content=body_link).

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

[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/WordPress/book?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=body_badge)

Happy chatting.


PS: [Click here](https://gitter.im/settings/badger/opt-out) if you would prefer not to receive automatic pull-requests from Gitter in future.