Tools: sitl_calibration: add note on using calibration model

This can avoid users having problems as reported at issue #4088 ("Calibration
not working in sitl").

Tools/mavproxy_modules/README.md

@@ -10,6 +10,10 @@ This module interfaces with the `calibration` model of SITL. It provides
 commands to actuate on the vehicle's rotation to simulate a calibration
 process.
 
+Make sure to pass `--model calibration` to the SITL binary in order to be able
+use this module's commands. You can also use
+`[sim_vehicle.py](../autotest/sim_vehicle.py)` with `--frame calibration`.
+
 ### Accelerometer Calibration ###
 The command `sitl_accelcal` listens to the accelerometer calibration status
 texts and set the vehicle in the desired attitude. Example: