AP_HAL_FLYMAPLE: Add notes to FlymaplePortingNotes.txt re GPS selection.

libraries/AP_HAL_FLYMAPLE/FlymaplePortingNotes.txt

@@ -297,6 +297,31 @@ You may (like me) need to reverse the Roll servo, using the Radio Calibration pa
 on Mission Planner, else the vehicle will steer in exactly the wrong direction
 when on Auto, but will be OK on Manual.
 
+GPS notes
+
+I tested intially with an EM-405A GPS (This is a 5Hz, 5V GPS, and therefore I
+also needed a voltage divider to make the received data compatible with the
+Flymaple 3.3V GPS input on pin D0).  This GPS was unsatisfactory due to long
+time lags in changing ground track and speed, and also due to large random
+ground speeds up to 1.0 m/s when stationary. I was able to get ok behaviour
+with very large values for NAVL1_PERIOD of around 40. Dont use this GPS.
+
+I also tested with a Eagle Tree GPS$v, which is a 10Hz, 3.3V GPS board
+containing a GTPA010 GPS. This GPS worked much better with the suggested
+tuning configuration from
+http://rover.ardupilot.com/wiki/tuning-steering-and-navigation-for-a-rover/
+
+For the record, the tuning I used with this was:
+
+CRUISE_SPEED	3 m/s
+CRUISE_THROTTLE	40 %
+NAVL1_DAMPING 	0.76
+NAVL1_PERIOD	10
+SPEED2THR_D	0.2
+SPEED2THR_I	0.2
+SPEED2THR_IMAX	4000
+SPEED2THR_P	0.7
+
 Remaining issues:
 
 1. Many alignment warnings emitted by the compiler from libraries/GCS_MAVLink