Add installation with operator docs

Author: Rulox

docs-web/installation/index.rst

@@ -7,4 +7,5 @@ Installation
    building-ingress-controller-image
    installation-with-manifests
    installation-with-helm
+   installation-with-operator
    running-multiple-ingress-controllers

docs-web/installation/installation-with-operator.md

@@ -0,0 +1,42 @@
+# Installation with the NGINX Ingress Operator
+
+This document describes how to install the NGINX Ingress Controller in your Kubernetes cluster using the NGINX Ingress Operator.
+
+## Prerequisites
+
+1. Make sure you have access to the Ingress Controller image:
+    * For NGINX Ingress Controller, use the image `nginx/nginx-ingress` from [DockerHub](https://hub.docker.com/r/nginx/nginx-ingress).
+    * For NGINX Plus Ingress Controller, build your own image and push it to your private Docker registry by following the instructions from [here](/nginx-ingress-controller/installation/building-ingress-controller-image).
+1. Install the NGINX Ingress Operator following the [instructions](https://github.com/nginxinc/nginx-ingress-operator/blob/master/docs/installation.md).
+
+## 1. Create the NginxIngressController manifest
+
+Create a manifest `nginx-ingress-controller.yaml` with the following content:
+
+```yaml
+apiVersion: k8s.nginx.org/v1alpha1
+kind: NginxIngressController
+metadata:
+  name: my-nginx-ingress-controller
+  namespace: default
+spec:
+  type: deployment
+  image:
+    repository: nginx/nginx-ingress
+    tag: edge
+    pullPolicy: Always
+  serviceType: NodePort
+  nginxPlus: False
+```
+
+**Note:** For NGINX Plus, change the `image.repository` and `image.tag` values and change `nginxPlus` to `True`.
+
+## 2. Create the NginxIngressController
+
+```
+$ kubectl apply -f nginx-ingress-controller.yaml
+```
+
+A new instance of the NGINX Ingress Controller will be deployed by the NGINX Ingress Operator in the `default` namespace with default parameters.
+
+To configure other parameters of the NginxIngressController resource, check the [documentation](https://github.com/nginxinc/nginx-ingress-operator/blob/master/docs/nginx-ingress-controller.md).