Add SAN docs (#552)

Author: ansd

docs/examples/tls/README.md

@@ -9,10 +9,27 @@ Assuming you already have these created and accessible as `server.pem` and `serv
 kubectl create secret tls tls-secret --cert=server.pem --key=server-key.pem
 ```
 
-This secret can also be created by [Cert Manager](https://cert-manager.io/).
+Alternatively, this secret can also be created by [Cert Manager](https://cert-manager.io/).
 
 Once the secret exists, you can deploy this example as follows:
 
 ```shell
 kubectl apply -f rabbitmq.yaml
 ```
+
+## SAN attributes for certificates
+
+Make sure that the certificate's Subject Alternative Name (SAN) contains at least the following attributes:
+* `*.<RabbitMQ cluster name>-nodes.<namespace>.svc.<K8s cluster domain name>`
+* `<RabbitMQ cluster name>.<namespace>.svc.<K8s cluster domain name>`
+
+If wildcards are not permitted, you must provide a SAN attribute for each RabbitMQ node in your RabbitMQ cluster.
+For example, if you deploy a 3-node RabbitMQ cluster named `myrabbit` in namespace `mynamespace` with the default Kubernetes cluster domain `cluster.local`, the SAN must include at least the following attributes:
+* `myrabbit-server-0.myrabbit-nodes.mynamespace.svc.cluster.local`
+* `myrabbit-server-1.myrabbit-nodes.mynamespace.svc.cluster.local`
+* `myrabbit-server-2.myrabbit-nodes.mynamespace.svc.cluster.local`
+* `myrabbit.mynamespace.svc.cluster.local`
+
+Note that the last SAN attribute is the client service DNS name.
+Depending on the service type you use (`spec.service.type`), you might need further SAN attributes.
+For example if you use service type `NodePort`, you need to include the external IP address of each K8s node to the SAN.