Expand ingress docs a bit (#1806)

This notes the controller intended to be used with the operator-managed ingress resources.

When setting up my own cluster I also tried https://docs.nginx.com/nginx-ingress-controller/ and https://kubernetes-sigs.github.io/aws-load-balancer-controller/ but the path format generated by the operator won't work with those.

docs/quick-start-guide.md

@@ -238,10 +238,20 @@ and deleting the pods outside the operator might lead to incorrect metric values
 ## Driver UI Access and Ingress
 
 The operator, by default, makes the Spark UI accessible by creating a service of type `ClusterIP` which exposes the UI. This is only accessible from within the cluster.
+
 The operator also supports creating an optional Ingress for the UI. This can be turned on by setting the `ingress-url-format` command-line flag. The `ingress-url-format` should be a template like `{{$appName}}.{ingress_suffix}/{{$appNamespace}}/{{$appName}}`. The `{ingress_suffix}` should be replaced by the user to indicate the cluster's ingress url and the operator will replace the `{{$appName}}` & `{{$appNamespace}}` with the appropriate value. Please note that Ingress support requires that cluster's ingress url routing is correctly set-up. For e.g. if the `ingress-url-format` is `{{$appName}}.ingress.cluster.com`, it requires that anything `*ingress.cluster.com` should be routed to the ingress-controller on the K8s cluster.
 
 The operator also sets both `WebUIAddress` which is accessible from within the cluster as well as `WebUIIngressAddress` as part of the `DriverInfo` field of the `SparkApplication`.
 
+The operator generates ingress resources intended for use with the [Ingress NGINX Controller](https://kubernetes.github.io/ingress-nginx/). Include this in your application spec for the controller to ensure it recognizes the ingress and provides appropriate routes to your Spark UI.
+
+```yaml
+spec:
+  sparkUIOptions:
+    ingressAnnotations:
+        kubernetes.io/ingress.class: nginx
+```
+
 ## About the Mutating Admission Webhook
 
 The Kubernetes Operator for Apache Spark comes with an optional mutating admission webhook for customizing Spark driver and executor pods based on the specification in `SparkApplication` objects, e.g., mounting user-specified ConfigMaps and volumes, and setting pod affinity/anti-affinity, and adding tolerations.