install-admission-controller should indicate how to check success

In the docs for setting up the admission controller, the `kubectl create -f admission.yaml` may appear to succeed, but a user wouldn't know for sure unless they run `kubectl get pods` and checked that the status got to `RUNNING`. In my particular case, it failed with ImagePullBackoff but I (luckily) knew to check.

Recommend adding a section checking for status after that create.

Output from mine after I fixed it is as such:

$ kubectl get pods

NAME                                           READY     STATUS    RESTARTS   AGE

couchbase-operator-admission-b97698c48-7fppd   1/1       Running   0          6m