Update Autopilot++ to v0.5.1

ICAO airports database was not included in the package -- added it in.
Also updated the link to the Github repository in the README.
Update Autopilot++ to v0.5.2

Fixed issue with PID controller gains causing instability in aircraft
which are highly responsive (small aircraft, aircraft at high speeds).
Also added information to README about how to use the plugin.
Add information on how to contribute

Well, we want people to be contributing, do we not?