Add upgrade documentation

Closes #2458

docs/deploy/upgrade.md

@@ -0,0 +1,50 @@
+# Upgrading
+
+!!! important
+    No matter the method you use for upgrading, *if you use template overrides,
+    make sure your templates are compatible with the new version of ingress-nginx*.
+
+## Without Helm
+
+To upgrade your ingress-nginx installation, it should be enough to change the version of the image
+in the controller Deployment.
+
+I.e. if your deployment resource looks like (partial example):
+
+```yaml
+kind: Deployment
+metadata:
+  name: nginx-ingress-controller
+  namespace: ingress-nginx
+spec:
+  replicas: 1
+  selector: ...
+  template:
+    metadata: ...
+    spec:
+      containers:
+        - name: nginx-ingress-controller
+          image: quay.io/kubernetes-ingress-controller/nginx-ingress-controller:0.9.0
+          args: ...
+```
+
+simply change the `0.9.0` tag to the version you wish to upgrade to.
+The easiest way to do this is e.g. (do note you may need to change the name parameter according to your installation):
+
+```
+kubectl set image deployment/nginx-ingress-controller \
+  nginx-ingress-controller=nginx:quay.io/kubernetes-ingress-controller/nginx-ingress-controller:0.14.0
+```
+
+For interactive editing, use `kubectl edit deployment nginx-ingress-controller`.
+
+
+## With Helm
+
+If you installed ingress-nginx using the Helm command in the deployment docs so its name is `ngx-ingress`,
+you should be able to upgrade using
+
+```shell
+helm upgrade --reuse-values ngx-ingress stable/nginx-ingress
+```
+