Update README.md

Some documentations

Author: Candas1

src/encoders/flux_observer/README.md

@@ -1,3 +1,39 @@
 # Flux Observer Sensor
 
-[MESC Book](https://davidmolony.github.io/MESC_Firmware/operation/CONTROL.html#the-sensorless-observer)
+The MXLEMMING has been ported from the MESC Firmware, it's also the default Flux observer in Vesc.
+The [MESC book](https://davidmolony.github.io/MESC_Firmware/operation/CONTROL.html#the-sensorless-observer) explains the math behind the flux observer.
+
+It's a simple solution for sensorless motor control only using phase currents and motor parameters, if tracking the position at low speed and when not driving the motor is not relevant.
+
+### Motor Parameters:
+The MXLEMMING Flux Observer needs the following motor parameters to be set:
+- phase resistance
+- KV rating
+- phase inductance
+- pole pairs
+
+It will not track the position if any of those parameters are missing.
+
+The KV rating and pole pairs parameters are used to derive the motor flux linkage ([code](https://github.com/Candas1/Arduino-FOC-drivers/blob/202ba0c4ae11558f6391c4bdff1e130a8735e178/src/encoders/flux_observer/FluxObserverSensor.cpp#L10)) which is key for the flux observer to run well.
+```
+BLDCMotor motor = BLDCMotor(15, 0.1664, 17.0, 0.00036858); // Hoverboard Motor
+```
+flux_linkage parameter can be adjusted from the code.
+
+### Current Sense
+The current sense is required as this flux observer only relies on phase currents.
+
+### Sensor Alignment
+The flux observer sensor doesn't need sensor alignment.
+```
+motor.sensor_direction= Direction::CW;
+motor.zero_electric_angle = 0;
+```
+
+### Bemf Threshold
+The sensor also has a bemf_threshold parameter (0 by default) that prevents the flux observer from tracking if the estimated bemf is not high enough ([code](https://github.com/Candas1/Arduino-FOC-drivers/blob/202ba0c4ae11558f6391c4bdff1e130a8735e178/src/encoders/flux_observer/FluxObserverSensor.cpp#L29)).
+This can help when starting the motor as the flux observer is not good at tracking the position at low speed.
+
+### To do:
+The Clarke transform is running both in the loopFOC and in the sensor update now, it can be remove from the sensor when the Alpha and Beta currents will be persisted as a BLDCMotor member.
+