Add example documentation for spline example

Author: urfeex

doc/examples.rst

@@ -22,5 +22,6 @@ may be running forever until manually stopped.
    examples/primary_pipeline_calibration
    examples/rtde_client
    examples/script_sender
+   examples/spline_example
    examples/tool_contact_example
    examples/ur_driver

doc/examples/spline_example.rst

@@ -0,0 +1,14 @@
+:github_url: https://github.com/UniversalRobots/Universal_Robots_Client_Library/blob/master/doc/examples/spline_example.rst
+
+.. _spline_example:
+
+Spline Interpolation example
+============================
+
+The URScript code inside `external_control.urscript
+<https://github.com/UniversalRobots/Universal_Robots_Client_Library/blob/master/resources/external_control.urscript>`_ implements trajectory execution based upon spline interpolation of a given sequence of waypoints.
+
+Depending on the waypoint parametrization, either cubic splines (positions, velocities, times) or
+quintic splines (positions, velocities, accelerations, times) will be used.
+
+The example in `spline_example.cpp <https://github.com/UniversalRobots/Universal_Robots_Client_Library/blob/master/examples/spline_example.cpp>`_ shows how to use the :ref:`trajectory_point_interface` to execute trajectories using this spline interpolation.