Update example with how to set credentials for kubeconfig

[carolynvs]

We don't call out that the kubeconfig should be in /home/nonroot/.kube/config or how to use the kubeconfig flag. Let's do that and add an example bundle (or link to one) so people have a working example.

[neeginotra]

@carolynvs @schristoff,
can you share the document link which we should update to add how to use a kubeconfig flag?
Are we talking about the Readme file kubernetes Mixin for Porter?