Docs for using Ambassador as Ingress Controller

This commit adds support for installing Ambassador
(https://getambassador.io/) as an Ingress Controller.

Ambassador is installed via the Ambassador Operator which
is the recommended way of installed Ambassador (it makes sure
users always have the latest version of Ambassador installed
and takes care of the update schedule as well).

site/content/docs/user/ingress.md

@@ -25,6 +25,7 @@ by the ingress controller `nodeSelector`.
 
 1. [Create a cluster](#create-cluster)
 2. Deploy an Ingress controller, the following ingress controllers are known to work:
+    - [Ambassador](#ambassador)
     - [Contour](#contour)
     - [Ingress NGINX](#ingress-nginx)
 
@@ -84,6 +85,38 @@ Refer to [Using Ingress](#using-ingress) for a basic example usage.
 
 Additional information about Contour can be found at: [projectcontour.io](https://projectcontour.io)
 
+### Ambassador
+
+[Ambassador OSS](https://www.getambassador.io/) will be installed with the help of
+the [Ambassador operator](https://www.getambassador.io/docs/latest/topics/install/aes-operator/).
+
+First install the CRDs with
+
+{{< codeFromInline lang="bash" >}}
+kubectl apply -f https://github.com/datawire/ambassador-operator/releases/latest/download/ambassador-operator-crds.yaml
+{{< /codeFromInline >}}
+
+Now install the kind-specific manifest for installing Ambassador OSS with the operator
+in the `ambassador` namespace:
+
+{{< codeFromInline lang="bash" >}}
+kubectl apply -n ambassador -f https://github.com/datawire/ambassador-operator/releases/latest/download/ambassador-operator-kind.yaml
+kubectl wait --timeout=180s -n ambassador --for=condition=deployed ambassadorinstallations/ambassador
+{{< /codeFromInline >}}
+
+Ambassador OSS is now ready for use. You can try the example in [Using Ingress](#using-ingress) at this moment,
+but Ambassador will not automatically load the `Ingress` defined there. `Ingress` resources must include
+the annotation `kubernetes.io/ingress.class: ambassador` for being recognized by Ambassador (otherwise they are just ignored).
+So once the example has been loaded you can add this annotation with a `patch`:
+
+{{< codeFromInline lang="bash" >}}
+kubectl patch ingress example-ingress -p '{"metadata":{"annotations":{"kubernetes.io/ingress.class": "ambassador"}}}'
+{{< /codeFromInline >}}
+
+Ambassador should be exposing your Ingress now. Please find additional documentation on
+Ambassador [here](https://www.getambassador.io/docs/latest/).
+
+
 ### Ingress NGINX
 
 {{< codeFromInline lang="bash" >}}