Label sample apps based on which architecture they will work for

[dneary]

I tried to validate my argocd set-up by installing the guestbook application from the sample apps - but it uses an amd64-only image from GCR. It would be great if all the sample apps used multiarch containers - but at a minimum, it would be nice if the sample applications clarified when they are arcitecture specific in https://github.com/argoproj/argocd-example-apps/blob/master/README.md