Add CustomResourceValidation example in sample-controller

- Mention the schema in the example CRD.
- Update README and mention about feature gates.

staging/src/k8s.io/sample-controller/README.md

@@ -73,6 +73,22 @@ type User struct {
 }
 ```
 
+## Validation
+
+To validate custom resources, use the [`CustomResourceValidation`](https://kubernetes.io/docs/tasks/access-kubernetes-api/extend-api-custom-resource-definitions/#validation) feature.
+
+This feature is beta and enabled by default in v1.9. If you are using v1.8, enable the feature using
+the `CustomResourceValidation` feature gate on the [kube-apiserver](https://kubernetes.io/docs/admin/kube-apiserver):
+
+```sh
+--feature-gates=CustomResourceValidation=true
+```
+
+### Example
+
+The schema in the [example CRD](./artifacts/examples/crd.yaml) applies the following validation on the custom resource:
+`spec.replicas` must be an integer and must have a minimum value of 1 and a maximum value of 10.
+
 ## Cleanup
 
 You can clean up the created CustomResourceDefinition with:

staging/src/k8s.io/sample-controller/artifacts/examples/crd.yaml

@@ -9,3 +9,12 @@ spec:
     kind: Foo
     plural: foos
   scope: Namespaced
+  validation:
+    openAPIV3Schema:
+      properties:
+        spec:
+          properties:
+            replicas:
+              type: integer
+              minimum: 1
+              maximum: 10