improve docu

docs/src/index.md

@@ -65,6 +65,12 @@ of the time step. The formulas are based on basic physics and aerodynamics and c
 
 ![Four point kite power system model](kps4.png)
 
+## Reference frames and control inputs
+- a positive `set_torque` will accelerate the reel-out, a negative `set_torque` counteract the pulling force of the kite. The unit is [N/m] as seen at the motor/generator axis.
+- the `depower` settings are dimensionless and can be between zero and one. A value equal to $\mathrm{depower\_zero}/100$ from the `settings.yaml` file means that the kite is fully powered. 
+
+A definition of the reference frames can be found [here](https://ufechner7.github.io/KiteUtils.jl/dev/reference_frames/) .
+
 ## Further reading
 The one point and four point kite models are described in detail in [Dynamic Model of a Pumping Kite Power System](http://arxiv.org/abs/1406.6218).