Add a case to illustrate how to use `kubectl get -o custom-columns`

pkg/kubectl/cmd/get/get.go

@@ -118,6 +118,9 @@ var (
 		# Return only the phase value of the specified pod.
 		kubectl get -o template pod/web-pod-13je7 --template={{.status.phase}}
 
+		# List resource information in custom columns.
+		kubectl get pod test-pod -o custom-columns=CONTAINER:.spec.containers[0].name,IMAGE:.spec.containers[0].image
+
 		# List all replication controllers and services together in ps output format.
 		kubectl get rc,services