[Examples] Update REAME.md with details on each example

examples/python/README.md

 # Pinocchio examples in Python
 
 This directory contains minimal examples on how to use **Pinocchio** with the Python bindings. 
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+# Loading a model
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+- Loading a URDF model: `python -i load-urdf.py`
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+# Computing analytical derivatives of rigid body dynamics algorithms
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+- Computing forward kinematics derivatives: `python -i kinematics-derivatives.py` 
+- Computing forward dynamics (fd) derivatives: `python -i fd-derivatives.py` 
+- Computing inverse dynamics (id) derivatives: `python -i id-derivatives.py`