[Ovmsdev] Readme rework

Everyone, I felt it was about time to polish our readme. Please have a look: https://github.com/openvehicles/Open-Vehicle-Monitoring-System-3 If I missed something that should be there, tell me. Thanks to Glyn for the flyer image :-) Regards, Michael -- Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal Fon 02333 / 833 5735 * Handy 0176 / 206 989 26