Update Supported Platforms section on Architecture document

[jkoby]

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Currently, the available architecture documentation at https://splunk.github.io/docker-splunk/ARCHITECTURE.html includes the following:

At the current time, this project only officially supports running Splunk Enterprise on debian:stretch-slim. We do have plans to incorporate other operating systems and Windows in the future.

Given that only RHEL-based images are being pushed to Docker Hub, this should be updated.

[alishamayor]

Thanks for pointing this out! Will update the documentation.