Add a Gitter chat badge to README.md

### w3c/csswg-test now has a Chat Room on Gitter

@snsk has just created a chat room. You can visit it here: [https://gitter.im/w3c/csswg-test](https://gitter.im/w3c/csswg-test?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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