Add a README in test/e2e/node with a warning

test/e2e/node/README.md

+# WARNING: Do not add tests in this directory
+
+There are two types of end-to-end tests in Kubernetes:
+ * [Cluster end-to-end tests](https://git.k8s.io/community/contributors/devel/e2e-tests.md)
+ * [Node end-to-end
+   tests](https://github.com/kubernetes/community/blob/master/contributors/devel/e2e-node-tests.md)
+
+Tests located in `${KUBE_ROOT}/test/e2e/common` are shared by both Cluster
+and Node E2E test jobs. Tests in `${KUBE_ROOT}/test/e2e_node` are exclusively
+owned by Node E2E. *If you want to add a test, most likely than not, you want
+to add the test to one of the two directories mentioned above.* If you are
+unsure, please check with the OWNER of the directory. This directory currently
+contains misplaced and legacy tests; they will be cleaned up in the future.