Add output interpretation section to Readme.md

Closes #2.

readme.md

@@ -34,3 +34,9 @@ As soon as it is initialized, the sensor will start accumulating motion count. C
   // Get motion count since last call
   flow.readMotionCount(&deltaX, &deltaY);
 ```
+
+## Output interpretation
+
+The sensor outputs unitless ticks that corresponds to the amount of movement that has been observed in front of the sensor. These ticks can be converted to a distance if we know the distance to the tracked surface and we assume the tracked surface is flat.
+
+Parameters of the PMW3901 sensor have been devised experimentally in order to implement it in the Crazyflie firmware. The measurement equation are described in  [Marcus Greiff's Master thesis](http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=8905295&fileOId=8905299) section 6.5. The implementation is part or the [Crazyflie kalman filter](https://github.com/bitcraze/crazyflie-firmware/blob/6308ff47ff4d4691f9b7f6f991564244c76d7910/src/modules/src/estimator_kalman.c#L1034-L1098).