Add cri-o document.

docs/cri-o.md

+cri-o
+===============
+
+cri-o is container developed by kubernetes project.
+Currently, only basic function is supported for cri-o.
+
+* cri-o is supported kubernetes 1.11.1 or later.
+* helm and other feature may not be supported due to docker dependency.
+* scale.yml and upgrade-cluster.yml are not supported.
+
+helm and other feature may not be supported due to docker dependency.
+
+Use cri-o instead of docker, set following variable:
+
+#### all.yml
+
+```
+kubeadm_enable: true
+...
+download_container: false
+skip_downloads: false
+```
+
+#### k8s-cluster.yml
+
+```
+etcd_deployment_type: host
+kubelet_deployment_type: host
+manage_docker: false
+manage_crio: true
+```
+