docs: clarify that this is a staging repository and not for direct contributions

Signed-off-by: sAchin-680 <[email protected]>

Kubernetes-commit: 7c43e6d2fdbf88f8d92559dac3cabb23da2a2957

Author: sAchin-680

README.md

@@ -1,3 +1,7 @@
+> ⚠️ **This is a staging repository 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.
+
 # apiextensions-apiserver
 
 Implements: https://github.com/kubernetes/design-proposals-archive/blob/main/api-machinery/thirdpartyresources.md