Add support options to README.md

[fejta]

/assign @erain @nlopezgi

Preview: https://github.com/fejta/rules_k8s/blob/doc/README.md#support

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @fejta. Thanks for your PR.

I'm waiting for a bazelbuild member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[fejta]

/ok-to-test