Document that RayClusters must be labeled with `app.kubernetes.io/name: kuberay` for kuberay-tpu-webhook

[davidxia]

In this README, it should be documented that RayClusters must be labeled with app.kubernetes.io/name: kuberay for kuberay-tpu-webhook to act on them. Mine didn't at first and it took me a while to realize without looking at source.