Add documentation.

doc/Doxyfile.extra.in

+INPUT                  = @PROJECT_SOURCE_DIR@/include \
+			 @PROJECT_SOURCE_DIR@/doc
+

doc/mainpage.hh

+/// \mainpage
+///
+/// \section Introduction
+///
+/// This package implements a bridge between ROS and the stack of tasks.
+/// The main features are
+///   \li A service <c>Run_command</c> that enables users to communicate with
+///       a remote python interpreter in order to build and control a graph,
+///   \li A class <c>dynamicgraph::RosRobotModel</c> deriving from
+///       <c>dynamicgraph::sot::Dynamic</c> that implements an entity computing
+///       forward kinematics and dynamics based on a urdf file.  for
+///       more information, type in a python terminal
+/// \code
+/// from dynamic_graph.ros.robot_model import RosRobotModel
+/// model = RosRobotModel ("model")
+/// model.help ()
+/// \endcode