Add some details about certs

content/en/docs/tasks/administer-cluster/kubeadm/kubeadm-certs.md

@@ -23,9 +23,12 @@ You should be familiar with [PKI certificates and requirements in Kubernetes](/d
 
 ## Renew certificates with the certificates API
 
-Kubeadm can renew certificates with the `kubeadm alpha certs renew` commands.
+The Kubernetes certificates reach normally their expiration date after 1 Year. 
+
+Kubeadm can renew certificates with the `kubeadm alpha certs renew` commands; you should run this command on Master only.
 
 Typically this is done by loading on-disk CA certificates and keys and using them to issue new certificates.
+
 This approach works well if your certificate tree is self-contained. However, if your certificates are externally
 managed, you might need a different approach.