Add link to downward api in downward example

examples/downward-api/README.md

 # Downward API example
 
 Following this example, you will create a pod with a containers that consumes the pod's name and
-namespace using the downward API.
+namespace using the [downward API](https://github.com/GoogleCloudPlatform/kubernetes/blob/master/docs/downward_api.md).
 
 ## Step Zero: Prerequisites