docs: ipv6: Add initial development guide

* Work in progress.

docs/ipv6.md

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+# IPv6 Support
+
+This is a work in progress, more polish to follow.  Use the `dev` git branch and `dev` docker image tag for testing.
+
+## Tunnel IPv6 Address To OpenVPN Clients
+
+This feature is advanced and recommended only for those who already have a functioning IPv4 tunnel and know how IPv6 works.
+
+Systemd is used to setup a static route and Debian 8.1 or later is recommended as the host distribution.  Others probably work, but haven't been tested.
+
+### Step 1 — Setup IPv6 on the Host Machine
+
+The tutorial uses a free tunnel from [tunnelbroker.net](https://tunnelbroker.net/) to get a /64 and /48 prefix allocated to me.  The tunnel endpoint is less then 3 ms away from Digital Ocean's San Francisco datacenter.
+
+Place the following in `/etc/network/interfaces`.  Relace `PUBLIC_IP` with your host's public IPv4 address and replace 2001:db8::2 and 2001:db8::1 with the corresponding tunnel endpoints:
+
+    auto he-ipv6
+    iface he-ipv6 inet6 v4tunnel
+        address 2001:db8::2
+        netmask 64
+        endpoint 72.52.104.74
+        local PUBLIC_IP
+        ttl 255
+        gateway 2001:db8::1
+
+Bring the interface up:
+
+    ifup he-ipv6
+
+Test that IPv6 works on the host:
+
+    ping6 google.com
+
+If this doesn't work, figure it out.  It may be necessary to add an firewall rule to allow IP protocol 41 through the firewall.
+
+### Step 2 — Setup the systemd Unit File
+
+Copy the systemd init file from the docker-openvpn /init directory of the repository and install into `/etc/systemd/system/docker-openvpn.conf`
+
+Edit the file, replace `IP6_PREFIX` value with the value of your /64 prefix.
+
+Finally, reload systemd so the changes take affect:
+
+    systemctl daemon-reload
+
+### Step 3 — Start OpenVPN
+
+Ensure that OpenVPN has been initialized and configured as described in the top level `README.md`.
+
+Start the systemd service file:
+
+    systemctl start docker-openvpn
+
+Verify logs if needed:
+
+    systemctl status docker-openvpn
+    docker logs openvpn0
+
+### Step 4 — Modify Client Config for IPv6 Default Route
+
+Append the default route for the public Internet:
+
+    echo "route-ipv6 2000::/3" >> clientname.ovpn
+
+### Step 5 — Start up Client
+
+If all went according to plan, then `ping6 2600::` and `ping6 google.com` should work.
+
+Fire up a web browser and attempt to navigate to [https://ipv6.google.com](https://ipv6.google.com).
+
+
+## Connect to the OpenVPN Server Over IPv6
+
+Not implemented, yet.