In kubeadm 1.10 (and perhaps earlier versions, not sure), ctrl-mgr and scheduler pods' label key is `component=` rather than `k8s-app=`, which is the selector key used for kube-prometheus exporters for these components.

One of the `sed` commands for updating the kubeadm systemd
file (`/etc/systemd/system/kubelet.service.d/10-kubeadm.conf`)
was missing a `-i` reference to the file itself, causing it
to hang indefinitely if ran as declared.

I also wrapped this second `sed` in an `if grep ...` check,
in order to make the operation idempotent.

Added information on webhook token authentication and information on how to update the manifests if Kubernetes is already deployed with kubeadm.

* 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