Added a mention of TEST_ONLY to extended test doc

HACKING.md

@@ -237,6 +237,11 @@ and can be run individually by specifying `--ginkgo.focus` and a regex filter:
 
     $ test/extended/core.sh --ginkgo.focus=<regex>
 
+In addition, the extended tests can be ran against an existing OpenShift
+cluster:
+
+    $ KUBECONFIG=/path/to/admin.kubeconfig TEST_ONLY=true test/extended/core.sh --ginkgo.focus=<regex>
+
 Extended tests should be Go tests in the `test/extended` directory that use
 the Ginkgo library. They must be able to be run remotely, and cannot depend on
 any local interaction with the filesystem or Docker.
@@ -245,7 +250,6 @@ More information about running extended tests can be found in
 [test/extended/README](https://github.com/openshift/origin/blob/master/test/exte
 nded/README.md).
 
-
 ## Installing Godep
 
 OpenShift and Kubernetes use [Godep](https://github.com/tools/godep) for