kubeadm-certs: add notes about different external CA approaches

There are multiple ways to prepare the credentials for use
with "external CA" mode:
- manual
- using kubeadm CSRs
- using kubeadm phases

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

@@ -46,8 +46,46 @@ CA key on disk.
 Instead, run the controller-manager standalone with `--controllers=csrsigner` and
 point to the CA certificate and key.
 
-[PKI certificates and requirements](/docs/setup/best-practices/certificates/) includes guidance on
-setting up a cluster to use an external CA.
+There are various ways to prepare the component credentials when using external CA mode.
+
+### Manual preparation of component credentials
+
+[PKI certificates and requirements](/docs/setup/best-practices/certificates/) includes information
+on how to prepare all the required by kubeadm component credentials manually.
+
+### Preparation of credentials by signing CSRs generated by kubeadm
+
+kubeadm can [generate CSR files](#signing-csr) that you can sign manually with tools like
+`openssl` and your external CA. These CSR files will include all the specification for credentials
+that components deployed by kubeadm require.
+
+### Automated preparation of component credentials by using kubeadm phases
+
+Alternatively, it is possible to use kubeadm phase commands to automate this process.
+
+- Go to a host that you want to prepare as a kubeadm control plane node with external CA.
+- Copy the external CA files `ca.crt` and `ca.key` that you have into `/etc/kubernetes/pki` on the node.
+- Prepare a temporary [kubeadm configuration file](/docs/reference/setup-tools/kubeadm/kubeadm-init/#config-file)
+called `config.yaml` that can be used with `kubeadm init`. Make sure that this file includes
+any relevant cluster wide or host-specific information that could be included in certificates, such as,
+`ClusterConfiguration.controlPlaneEndpoint`, `ClusterConfiguration.certSANs` and `InitConfiguration.APIEndpoint`.
+- On the same host execute the commands `kubeadm init phase kubeconfig all --config config.yaml` and
+`kubeadm init phase certs all --config config.yaml`. This will generate all required kubeconfig
+files and certificates under `/etc/kubernetes/` and its `pki` sub directory.
+- Inspect the generated files. Delete `/etc/kubernetes/pki/ca.key`, delete or move to a safe location
+the file `/etc/kubernetes/super-admin.conf`.
+- On nodes where `kubeadm join` will be called also delete `/etc/kubernetes/kubelet.conf`.
+This file is only required on the first node where `kubeadm init` will be called.
+- Note that some files such `pki/sa.*`, `pki/front-proxy-ca.*` and `pki/etc/ca.*` are
+shared between control plane nodes, You can generate them once and
+[distribute them manually](/docs/setup/production-environment/tools/kubeadm/high-availability/#manual-certs)
+to nodes where `kubeadm join` will be called, or you can use the
+[`--upload-certs`](/docs/setup/production-environment/tools/kubeadm/high-availability/#stacked-control-plane-and-etcd-nodes)
+functionality of `kubeadm init` and `--certificate-key` of `kubeadm join` to automate this distribution.
+
+Once the credentials are prepared on all nodes, call `kubeadm init` and `kubeadm join` for these nodes to
+join the cluster. kubeadm will use the existing kubeconfig and certificate files under `/etc/kubernetes/`
+and its `pki` sub directory.
 
 ## Check certificate expiration