Document support matrix for kubetestl and kubernetes client versions vs K8s versions

[timblaktu]

From #220, I learned that the API versions supported by kubetest objects appear to be hard-coded to the api-versions of a particular kubernetes release, but this is not documented and from the prevalence of related issues appears to be error prone.

I would like to see this information written down somewhere so I can know what to expect when I run kubetest against different version clusters.

There is the compatibility matrix for the kubernetes python client which maps which client package version corresponds to a kubernetes version.

There is also the kubernetes package requirement specified in this repo's setup.py.

What I'm asking for here is some clarity in the form of some docs that collect these details and state what kubernetes version is supported by each version of kubetest, and what kubernetes client package version that corresponds to. Similar to the ask in #219.

In a follow up issue I could propose a different way around this issue: generate collections of versioned kubetest objects that correspond to each version of Kubernetes, and allow kubetest to dynamically select the correct kubetest objects at runtime based on discovery or explicit K8s version specified by a new --kubernetes-version pytest arg.