Add a Gitter chat badge to README.md

### amousset/rust-smtp now has a Chat Room on Gitter

@amousset has just created a chat room. You can visit it here: [https://gitter.im/amousset/rust-smtp](https://gitter.im/amousset/rust-smtp?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%20Chat.svg)](https://gitter.im/amousset/rust-smtp?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=body_badge)

Happy chatting.


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