Document what fault cause the motor board fault LED to light

This draws its information from #98
but starts afresh to effect a more minimal change.

Fixes #96

Author: PeterJCLaw

kit/motor_board.md

@@ -34,7 +34,7 @@ Indicators
 
 | LED                    | Meaning                 | Initial power-up state
 |------------------------|-------------------------|------------------------------
-| Power                  | The board is powered    | On
+| Power                  | Green when powered correctly<br>Red when polarity is wrong for Power In | Green
 | M{0,1} Speed/Direction | Brightness indicates speed, colour indicates direction | Off
 | USB Power              | The USB interface is powered | On
 | USB Data               | Data is being transferred to/from the board | Off