Add pinout for mag connector (RTF and Compute Board Breakout)

[gjkunde]

While the aeroboard writeup correctly states the pinout for
the compass header on the breakout board and has a warning
in red that the battery voltage is patched through to Vcc, it could in
addition state that the compass header on the RTF drone and compass
header on the breakout board have opposite wiring - not exactly what we expected.
They are mirror images ... make developing more difficult ....