add reference_frames.md

Author: ufechner7

docs/src/reference_frames.md

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+## Reference Frames
+
+### Introduction
+Reference frames are needed for following purposes:
+- for creating a CAD model of the wing (or the wings)
+- for defining the apparent wind speed vector $v_a$
+- for calculating the lift and drag and side force coefficients
+- for calculating the resulting forces and moments
+
+### CAD reference frame (CAD)
+A geometric model is always created using the CAD reference frame.
+It can have any origin (with respect to the kite), but usually either the center of gravity of the body or the bridle point/ Kite Control Unit is used. 
+
+### Kite reference frame (K)
+This is a body-fixed reference frame.