Add tip about the --version flag to error output

[Tenzer]

If people are trying to use the Docker image to check their cluster, there's a
big likelyhood of them hitting the error message saying that either kubectl
or kubelet need to be found in order for kube-bench to be able to determine
the Kubernetes version in use.

This adds a tip that the version can be specified manually with the --version
flag which is a lot easier than having to make a new Docker image with the
right version of kubelet/kubectl in order for kube-bench to work.

[lizrice]

Good idea, thanks!