Add a Gitter chat badge to README.md

### aki017/slack-ruby-gem now has a Chat Room on Gitter

@aki017 has just created a chat room. You can visit it here: [https://gitter.im/aki017/slack-ruby-gem](https://gitter.im/aki017/slack-ruby-gem?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&content=body_link).

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

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Happy chatting.


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