README: Explicitly create volume container

* Implicit creation seems to have issues.
* Resolves issue #175

Author: kylemanna

README.md

@@ -24,6 +24,7 @@ a corresponding [Digital Ocean Community Tutorial](http://bit.ly/1AGUZkq).
 
 * Initialize the `$OVPN_DATA` container that will hold the configuration files and certificates
 
+        docker volume create --name $OVPN_DATA
         docker run -v $OVPN_DATA:/etc/openvpn --rm kylemanna/openvpn ovpn_genconfig -u udp://VPN.SERVERNAME.COM
         docker run -v $OVPN_DATA:/etc/openvpn --rm -it kylemanna/openvpn ovpn_initpki