add pointer to CONTRIBUTING.md for more details on contributing, clarify read-only

Kubernetes-commit: ada2ed8487708056ff11bd2413da1073558e6d7c

Author: BenTheElder

README.md

@@ -1,6 +1,7 @@
 > ⚠️ **This is a [staging repository](https://git.k8s.io/kubernetes/staging#external-repository-staging-area) for Kubernetes**.  
 > Contributions, including issues and pull requests, should be made to the main Kubernetes repository: [https://github.com/kubernetes/kubernetes](https://github.com/kubernetes/kubernetes).  
-> This repository is not for direct contributions.
+> This repository is read-only for importing, and not used for direct contributions.  
+> See [CONTRIBUTING.md](./CONTRIBUTING.md) for more details.
 
 # apiextensions-apiserver