Add auth-signin annotation

examples/external-auth/README.md

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+## External Authentication
+
+### Overview
+
+The `auth-url` and `auth-signin` annotations allow you to use an external
+authentication provider to protect your Ingress resources.
+
+(Note, this annotation requires `nginx-ingress-controller v0.9.0` or greater.)
+
+### Key Detail
+
+This functionality is enabled by deploying multiple Ingress objects for a single host.
+One Ingress object has no special annotations and handles authentication.
+
+Other Ingress objects can then be annotated in such a way that require the user to
+authenticate against the first Ingress's endpoint, and can redirect `401`s to the
+same endpoint.
+
+Sample:
+
+```
+...
+metadata:
+  name: application
+  annotations:
+    "ingress.kubernetes.io/auth-url": "https://$host/oauth2/auth"
+    "ingress.kubernetes.io/signin-url": "https://$host/oauth2/sign_in"
+...
+```
+
+### Example: OAuth2 Proxy + Kubernetes-Dashboard
+
+This example will show you how to deploy [`oauth2_proxy`](https://github.com/bitly/oauth2_proxy)
+into a Kubernetes cluster and use it to protect the Kubernetes Dashboard.
+
+#### Prepare:
+
+1. `export DOMAIN="somedomain.io"`
+2. Install `nginx-ingress`. If you haven't already, consider using `helm`: `$ helm install stable/nginx-ingress`
+3. Make sure you have a TLS cert added as a Secret named `ingress-tls` that corresponds to your `$DOMAIN`.
+
+### Deploy: `oauth2_proxy`
+
+This is the Deployment object that runs `oauth2_proxy`.
+