Make k8s.io/kubernetes dependency policy explicit

Author: liggitt

README.md

@@ -31,6 +31,9 @@ Try our [interactive tutorial].
 
 Take a free course on [Scalable Microservices with Kubernetes].
 
+To use Kubernetes code as a library in other applications, see the [list of published components](https://git.k8s.io/kubernetes/staging/README.md).
+Use of the `k8s.io/kubernetes` module or `k8s.io/kubernetes/...` packages as libraries is not supported.
+
 ## To start developing Kubernetes
 
 The [community repository] hosts all information about