Automatic merge from submit-queue

Adds docs for dns-horizontal-autoscaler and kube-dns

Although we have separate docs on kubernetes.io, we should have a short description about the dns-horizontal-autoscaler addon in folder.

Also updates kube-dns README with example command to scale kube-dns Deployment. This is needed because Addon Manager v6 has stricter reconcile behavior.

@bowei @bprashanth @thockin