Add CustomResourceValidation example in sample-controller

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

Kubernetes-commit: 74c9efa148ac6591a1dcf8f95c7fdac35aa603d0

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