Add blue-green deployment example (#19)

blue-green/README.md

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+# Blue Green
+
+The blue green strategy is not supported by built-in Kubernetes Deployment but available via third-party Kubernetes controller.
+This example demonstrates how to implement blue-green deployment via [Argo Rollouts](https://github.com/argoproj/argo-rollouts):
+
+1. Install Argo Rollouts controller: https://github.com/argoproj/argo-rollouts#installation
+2. Create a sample application and sync it.
+
+```
+argocd app create --name blue-green --repo https://github.com/argoproj/argocd-example-apps --dest-server https://kubernetes.default.svc --dest-namespace default --path blue-green && argocd app sync blue-green
+```
+
+Once the application is synced you can access it using `blue-green-helm-guestbook` service.
+
+3. Change image version parameter to trigger blue-green deployment process:
+
+```
+argocd app set blue-green -p image.tag=0.2 && argocd app sync blue-green
+```
+
+Now application runs `ks-guestbook-demo:0.1` and `ks-guestbook-demo:0.2` images simultaneously.
+The `ks-guestbook-demo:0.2` is still considered `blue` available only via preview service `blue-green-helm-guestbook-preview`.
+
+4. Promote `ks-guestbook-demo:0.2` to `green` by patching `Rollout` resource:
+
+```
+argocd app patch-resource blue-green --kind Rollout --resource-name blue-green-helm-guestbook --patch '{ "spec": { "paused": false } }' --patch-type 'application/merge-patch+json'
+```
+
+This promotes `ks-guestbook-demo:0.2` to `green` status and `Rollout` deletes old replica which runs `ks-guestbook-demo:0.1`.