Update Curve.derivatives() docstring

Author: Onur Rauf Bingol

geomdl/BSpline.py

@@ -199,6 +199,19 @@ def evaluate_list(self, param_list):
     def derivatives(self, u, order=0, **kwargs):
         """ Evaluates n-th order curve derivatives at the given parameter value.
 
+        The output of this method is list of n-th order derivatives. If ``order`` is ``0``, then it will only output
+        the evaluated point. Similarly, if ``order`` is ``2``, then it will output the evaluated point, 1st derivative
+        and the 2nd derivative. For instance;
+
+        .. code-block:: python
+
+            # Assuming a curve (crv) is defined on a parametric domain [0.0, 1.0]
+            # Let's take the curve derivative at the parametric position u = 0.35
+            ders = crv.derivatives(u=0.35, order=2)
+            ders[0]  # evaluated point, equal to crv.evaluate_single(0.35)
+            ders[1]  # 1st derivative at u = 0.35
+            ders[2]  @ 2nd derivative at u = 0.35
+
         :param u: parameter value
         :type u: float
         :param order: derivative order