docs: Add systemd documentation

* Describe how to use the reference service.
* Closes #200

init/docker-openvpn@.service

@@ -1,3 +1,35 @@
+#
+# Docker + OpenVPN systemd service
+#
+# Author: Kyle Manna <kyle@kylemanna.com>
+# Source: https://github.com/kylemanna/docker-openvpn
+#
+# This service aims to make the update and invocation of the docker-openvpn
+# container seemless.  It automatically downloads the latest docker-openvpn
+# image and instantiates a Docker container with that image.  At shutdown it
+# cleans-up the old container.
+#
+# In the event the service dies (crashes, or is killed) systemd will attempt
+# to restart the service every 10 seconds until the service is stopped with
+# `systemctl stop docker-openvpn@NAME`.
+#
+# A number of IPv6 hacks are incorporated to workaround Docker shortcomings and
+# are harmless for those not using IPv6.
+#
+# To use:
+# 1. Create a Docker volume container named `ovpn-data-NAME` where NAME is the
+#    user's choice to describe the use of the container.
+# 2. Initialize the data container according to the docker-openvpn README, but
+#    don't start the container. Stop the docker container if started.
+# 3. Download this service file to /etc/systemd/service/docker-openvpn@.service
+# 4. Enable and start the service template with:
+#    `systemctl enable --now docker-openvpn@NAME.service`
+# 5. Verify service start-up with:
+#    `systemctl status docker-openvpn@NAME.service`
+#    `journalctl --unit docker-openvpn@NAME.service`
+# 
+# For more information, see the systemd manual pages.
+#
 [Unit]
 Description=OpenVPN Docker Container
 Documentation=https://github.com/kylemanna/docker-openvpn