Add guidelines to write good conformance tests for k8s

[tonifinger]

This PR contains guidelines and best practices on how to write good conformance tests for K8S by using Sonobuoy.

[mbuechse]

Thanks for drafting this! I think we should address whatever Markdownlint says. Apart from that, I would ask you to be more brief and to the point. I suggest to use simple sentences, such as "First, we check whether all requirements of the development environment are met" (instead of "this can be used to") or "Next, we start the development Kind cluster" (instead of "Once ..., you can ..."). This kind of holding-hands step-by-step guide is appropriate, but I think we should also have a brief(!) descriptive overview as well. For instance, explain the structure of the test directory, the components that are involved in testing etc.