Document how to set up a compatible cluster

[imjasonh]

Based on https://github.com/knative/serving/blob/master/docs/creating-a-kubernetes-cluster.md or at a minimum link to that documentation.

/kind doc

[knative-housekeeping-robot]

Issues go stale after 90d of inactivity. Mark the issue as fresh with /remove-lifecycle stale. Stale issues rot after an additional 30d of inactivity and eventually close. If this issue is safe to close now please do so with /close. Send feedback to Knative Productivity Slack channel or knative/test-infra. /lifecycle stale