Documentation: Added kubeadm documentation #618 (#770)

* Documentation: Added kubeadm documentation #618

This getting started guide covers a simple deployment of kube-prometheus
and assumes you have deployed using kubeadm.

Hopefully this helps clears up #618

* Moved documentation to appropriate folder, updated links.

* Minor documentation editing

Fixed referneces to Alertmanager.
Slightly stronger wording on changing grafana user/pass