Add document outlining upgrade process

docs/upgrades.md

+Upgrading Kubernetes in Kargo
+=============================
+
+#### Description
+
+Kargo handles upgrades the same way it handles initial deployment. That is to
+say that each component is laid down in a fixed order. You should be able to
+upgrade from Kargo tag 2.0 up to the current master without difficulty. You can
+also individually control versions of components by explicitly defining their
+versions. Here are all version vars for each component:
+
+* docker_version
+* kube_version
+* etcd_version
+* calico_version
+* calico_cni_version
+* weave_version
+* flannel_version
+* kubedns_version
+
+#### Example
+
+If you wanted to upgrade just kube_version from v1.4.3 to v1.4.6, you could
+deploy the following way:
+
+```
+ansible-playbook cluster.yml -i inventory/inventory.cfg -e kube_version=v1.4.3
+```
+
+And then repeat with v1.4.6 as kube_version:
+
+```
+ansible-playbook cluster.yml -i inventory/inventory.cfg -e kube_version=v1.4.6
+```
+
+#### Upgrade order
+
+As mentioned above, components are upgraded in the order in which they were
+installed in the Ansible playbook. The order of component installation is as
+follows:
+
+# Docker
+# etcd
+# kubelet and kube-proxy
+# network_plugin (such as Calico or Weave)
+# kube-apiserver, kube-scheduler, and kube-controller-manager
+# Add-ons (such as KubeDNS)