Add a Gitter chat badge to readme.md

### richorama/AzureArchitectureReference now has a Chat Room on Gitter

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

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