kubeadm-upgrade-1.13: add workaround for join control-plane (#12145)

content/en/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade-1-13.md

@@ -29,6 +29,12 @@ This page explains how to upgrade a Kubernetes cluster created with `kubeadm` fr
   That is, you cannot skip versions when you upgrade.
   For example, you can upgrade only from 1.10 to 1.11, not from 1.9 to 1.11.
 
+{{< warning >}}
+The command `join --experimental-control-plane` is known to fail on single node clusters created with kubeadm v1.12 and then upgraded to v1.13.x.
+This will be fixed when graduating the `join --control-plane` workflow from alpha to beta.
+A possible workaround is described [here](https://github.com/kubernetes/kubeadm/issues/1269#issuecomment-441116249).
+{{</ warning >}}
+
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