Add link to example for CRDs (#5641)

* Add link to example for CRDs

In the CustomResourceDefinitions section, add a link to the [Custom Resource Example](https://github.com/kubernetes/kubernetes/tree/master/staging/src/k8s.io/apiextensions-apiserver/examples/client-go).

* add note

docs/concepts/api-extension/custom-resources.md

@@ -53,8 +53,12 @@ This frees you from writing your own API server to handle the custom resource,
 but the generic nature of the implementation means you have less flexibility than with
 [API server aggregation](#api-server-aggregation).
 
-CRD is the successor to the deprecated *ThirdPartyResource* (TPR) API, and is available as of
-Kubernetes 1.7.
+Refer to the [Custom Resource Example](https://github.com/kubernetes/kubernetes/tree/master/staging/src/k8s.io/apiextensions-apiserver/examples/client-go)
+for a demonstration of how to register a new custom resource, work with instances of your new resource type,
+and setup a controller to handle events.
+
+**Note:** CRD is the successor to the deprecated *ThirdPartyResource* (TPR) API, and is available as of Kubernetes 1.7.
+{: .note}
 
 ## API server aggregation