README file changes - improvements to Kubernetes instructions

[jkomg]

Comments from Lightbend

StatefulSet should specify "imagePullPolicy: IfNotPresent" instead of "Always"
if they intend for the example to work with Minikube.

It would be helpful to have some more comments about prereqs, kubectl, minikube and so on.

Again, if providing instructions to run locally on minikube, you should mention that you need to run "eval$(minikube docker-env)" before building the application