docs: add minimal usage info

Documentation/using-docker-skim.md

@@ -0,0 +1,77 @@
+# Using docker-skim
+
+**Note: running the docker daemon with this tool is not supported and may break with no notice**.
+
+## Using docker-skim to run docker 1.12
+
+On a Container Linux machine, setup the following systemd unit for docker.
+Override the existing unit by placing it at the path
+`/etc/systemd/system/docker.service`.
+
+If this is done on a running machine, rather than during provisioning via Ignition, first stop the existing docker and containerd units.
+
+```ini
+[Unit]
+Description=Docker Application Container Engine
+Documentation=http://docs.docker.com
+After=docker.socket
+Requires=docker.socket
+
+[Service]
+Type=simple
+
+ExecStart=/usr/bin/rkt run --dns=host --interactive \
+  --trust-keys-from-https \
+  --stage1-name=users.developer.core-os.net/skim/stage1-skim:0.0.1 \
+  users.developer.core-os.net/skim/docker:1.12.6_coreos.0 \
+  --exec=/usr/lib/coreos/dockerd -- \
+  --host=fd:// $DOCKER_OPTS $DOCKER_CGROUPS $DOCKER_OPT_BIP $DOCKER_OPT_MTU $DOCKER_OPT_IPMASQ
+
+ExecReload=/bin/kill -s HUP $MAINPID
+LimitNOFILE=1048576
+LimitNPROC=infinity
+LimitCORE=infinity
+TasksMax=infinity
+TimeoutStartSec=0
+# set delegate yes so that systemd does not reset the cgroups of docker containers
+Delegate=yes
+
+[Install]
+WantedBy=multi-user.target
+```
+
+Afterwards, run `systemctl daemon-reload` and `systemctl start docker` per usual.
+
+### The docker CLI
+
+The `docker` command will, by default, be the client version shipped in the host operating system. Generally, this CLI will work with older docker daemon versions.
+
+In order to use a matching `docker` cli version, the following script can be used as the docker command (e.g. placed in `/opt/bin/docker` and added to a user's path):
+
+```sh
+#!/bin/bash
+
+[[ -f /etc/coreos/docker.conf ]] && source /etc/coreos/docker.conf
+
+# The docker command is only accessible if the service is running
+
+if [ ! -x "/usr/bin/rkt" ]; then
+	echo "rkt is not installed"
+	exit 254
+fi
+
+RKTPODBASE="/var/lib/rkt/pods/run"
+STAGE2BRIDGE="/stage1/rootfs/opt/stage2/"
+
+ROUT=`rkt list --format=json`
+
+if [ "x${ROUT}" == "xnull" ]; then
+	echo "Docker cannot be started"
+	exit 254
+fi
+
+UUIDNAME=`echo ${ROUT}|jq -r '.[]|select(.state == "running" and (.app_names[]|contains("docker")))|.name'`
+
+DOCKER="${RKTPODBASE}/${UUIDNAME}/${STAGE2BRIDGE}/docker/rootfs/usr/bin/docker"
+exec ${DOCKER} "$@"
+```