Add Production-ready druid cluster spec

[schmichri]

This shall help users to adapt to the setup and config of Druid on Kubernetes using your operator. Due to the current lack of examples I've posted this on the druid slack channel and @AdheipSingh thought it would be of value here.

Description

In Context of the fahrbar20 project from iunera for the German Ministry of Digital and Transport, we decided to publish all our code and findings. Furthermore we love Druid and want to help with the adaption of users.

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This PR has:

• been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
• been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

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Key changed/added files in this PR

• docs/README.md