Add docs for kops, explain when kops and when kubeadm

Quick getting started guide for kops.

Also try to provide some guidance as to when to use kops and when
kubeadm; based on discussions with sig-cluster-lifecycle that kubeadm is
a building block and not a provisioning tool.

I used "kubeadm" as a simplifying concept for "kubeadm and the other
work done by sig-cluster-lifecycle that kubeadm is a part of", in that
tools that don't (yet) use kubeadm are still leveraging the kubeadm
stream of work.

docs/getting-started-guides/kubeadm.md

@@ -21,7 +21,20 @@ It is simple enough that you can easily integrate its use into your own automati
 
 See the full [`kubeadm` reference](/docs/admin/kubeadm) for information on all `kubeadm` command-line flags and for advice on automating `kubeadm` itself.
 
-**The `kubeadm` tool is currently in alpha but please try it out and give us [feedback](/docs/getting-started-guides/kubeadm/#feedback)!**
+**The `kubeadm` tool is currently in alpha but please try it out and give us [feedback](/docs/getting-started-guides/kubeadm/#feedback)!
+Be sure to read the [limitations](#limitations); in particular note that kubeadm doesn't have great support for
+automatically configuring cloud providers.  Please refer to the specific cloud provider documentation or
+use another provisioning system.**
+
+kubeadm assumes you have a set of machines (virtual or real) that are up and running.  It is designed
+to be part of a larger provisioning system - or just for easy manual provisioning.  kubeadm is a great
+choice where you have your own infrastructure (e.g. bare metal), or where you have an existing
+orchestration system (e.g. Puppet) that you have to integrate with.
+
+If you are not constrained, other tools build on kubeadm to give you complete clusters:
+
+* On GCE, [Google Container Engine](https://cloud.google.com/container-engine/) gives you turn-key Kubernetes
+* On AWS, [kops](kops) makes installation and cluster management easy (and supports high availability)
 
 ## Prerequisites