Merge pull request #29785 from petemiron/patch-1

Automatic merge from submit-queue

Clarify documentation that new admission control plugin must be imported

```release-note
```

Admission control design doc doesn't mention importing the plugin to plugins.go. I was unable to get the plugin to build into my binary without it. Updated documentation to prevent future confusion.

docs/design/admission_control.md

@@ -104,6 +104,17 @@ func init() {
 }
 ```
 
+A **plug-in** must be added to the imports in [plugins.go](../../cmd/kube-apiserver/app/plugins.go)
+
+```go
+  // Admission policies
+  _ "k8s.io/kubernetes/plugin/pkg/admission/admit"
+  _ "k8s.io/kubernetes/plugin/pkg/admission/alwayspullimages"
+  _ "k8s.io/kubernetes/plugin/pkg/admission/antiaffinity"
+  ...
+  _ "<YOUR NEW PLUGIN>"
+```
+
 Invocation of admission control is handled by the **APIServer** and not
 individual **RESTStorage** implementations.