Add a Gitter chat badge to README.md

### spring-cloud/spring-cloud-security now has a Chat Room on Gitter

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

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