Document how to use Pod Security Policies

According to @chrislovecnm , the Operator works with [Pod Security Policies](https://github.com/cockroachdb/cockroach-operator/issues/216)

The [official docs](https://kubernetes.io/docs/concepts/policy/pod-security-policy/) state: "Pod Security Policies enable fine-grained authorization of pod creation and updates."

One of our big customers requested support for PSPs.

Let's include a note in how to use it.

@keith-mcclellan , given that PSPs allow users to have more fine-grained authorization. Is this going to be related to our work in addressing the permissions bug ([Address Permissions Required by the Operator](https://github.com/cockroachdb/cockroach-operator/issues/258))?