Describe algorithm in whitepaper

GodotXRIKRig/docs/whitepaper.md

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+# How the inverse kinematics is calculated
+
+
+
+The concept uses the following assumptions:
+  * The degrees-of-freedom are limited to 
+    * Shoulder joint with 3dof and restrictive joint limits
+    * The elbow with only 1dof
+    * The wrist with 3dof and restrictive joint limits
+  * for humans, within the reachable elbow positions, some areas are more likely to be used due to less muscle strain
+
+## Proposal 1 for am pose estimate
+
+Input:
+  * controller position
+  * shoulder position estimate
+
+Output:
+  * Elbow position
+  * Shoulder backslash (required to improve shoulder position estimate)
+
+### Step 1: Initial elbow position estimate
+
+We try to get a first good estimate using a very simple method: given 
+  * a vector *SpineShoulder*, going from the spine at approx. breast-hight to the shoulder position, and
+  * a vector *ShoulderWrist*, going from the shoulder to the wrist position, which can be easily inferred from the controller position,
+  * calculate the cross-product like this for the **right shoulder**:
+  ```var elbow_dir = SpineShoulder.cross(ShoulderWrist)```.
+
+### Step 2: Consider controller orientation
+
+
+### Step 3: Consider shoulder orientation
+
+
+### Step 4: Determine shoulder backslash for shoulder position estimation
+