docs: external dns config

Author: natthan-pigoux

docs/RUN_PROD.md

@@ -117,6 +117,18 @@ ingress:
   className: "traefik"
 ```
 
+### ExternalDNS
+
+If your cluster provides [external-dns](https://github.com/kubernetes-sigs/external-dns) to expose ingresses and services, you can configured the ingress using a prefered IP (probably provided by [kube-vip](https://kube-vip.io/) ). Usually set the IP on the ingress and external DNS should automatically creates the entry otherwise you can add the following annotations:
+
+```yaml
+ingress:
+  annotations:
+    external-dns.alpha.kubernetes.io/hostname: "my.hostname"
+    external-dns.alpha.kubernetes.io/target: "prefered-ip"
+  loadBalancerIP: "prefered-ip"
+```
+
 ## Sandbox
 
 The Sandbox needs to be an object store. We highly recommend that you use one provided by your institute.