Update readme

[fjy]

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[gianm]

@fjy could you include the incubation disclaimer too (https://incubator.apache.org/guides/branding.html#disclaimers)? Perhaps also some of the same explanatory language on https://druid.apache.org/.

The disclaimer is:

Disclaimer: Apache Druid is an effort undergoing incubation at The Apache Software Foundation (ASF), sponsored by the Apache Incubator. Incubation is required of all newly accepted projects until a further review indicates that the infrastructure, communications, and decision making process have stabilized in a manner consistent with other successful ASF projects. While incubation status is not necessarily a reflection of the completeness or stability of the code, it does indicate that the project has yet to be fully endorsed by the ASF.

[fjy]

@gianm added disclaimer

[gianm]

LGTM. Would you like to include any of the other language from https://druid.apache.org/?

[fjy]

@gianm I think we should try to direct people to the Druid webpage as much as possible and avoid repeating information in too many places.