document nginx ingress (skupper-440) (skupperproject#253)

* document nginx ingress (skupper-440)

cli/index.adoc

@@ -452,6 +452,8 @@ include::./single-logical-service.adoc[leveloffset=0]
 
 include::./protocols.adoc[leveloffset=0]
 
+include::../kubernetes/ingress.adoc[leveloffset=1]
+
 // Type: reference
 [id="cli-global-options"] 
 == CLI options

kubernetes/ingress.adoc

@@ -0,0 +1,37 @@
+include::../partials/attributes.adoc[]
+// Type: concept
+[id="understanding-ingress"] 
+= Understanding ingress
+
+When you create a site that can be linked to, you need to enable ingress on that site.
+By default, ingress is enabled, however you can disable it or set it to use a specific ingress type.
+
+By default, the ingress type is set to:
+
+* `route` if available (OpenShift)
+* `loadbalancer`
+
+Other options include:
+
+* `none` useful if you do not need to link to the current site.
+* `nodeport`
+* `nginx-ingress-v1`
+* `contour-http-proxy`
+
+You can set the ingress type using the CLI when creating the site `skupper init --ingress <type>` or by setting the type in your site YAML, for example to disable ingress:
+
+[source, bash]
+----
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: skupper-site
+data:
+  name: my-site
+  ingress: "none"
+----
+
+If the default ingress is not suitable, an alternative is `nginx-ingress-v1`.
+Nginx uses Server Name Indication (SNI) to identify connection targets, which eliminates the need for assigning separate IP addresses as required by `loadbalancer`.
+
+NOTE: When using `nginx-ingress-v1` you must enable SSL Passthrough as described in link:https://kubernetes.github.io/ingress-nginx/user-guide/tls/#ssl-passthrough[Ingress-Nginx Controller documentation].