ingress: Show how to import cert as secret

ingress/README.md

@@ -8,6 +8,9 @@ their addresses)
 
 ```sh
 kubectl label node k8s-amd64-n{0,1,2}.pyrocufflink.blue kubernetes.io/role=ingress
+kubectl create secret tls pyrocufflink-cert \
+    --cert ~/certs/certificates/_.pyrocufflink.net.crt \
+    --key ~/certs/certificates/_.pyrocufflink.net.key
 kubectl apply -f ingress-nginx.yaml
 ```