cmd_vel remapping for rbx1 compatibility

[roboticslab-fr]

Remapping is mandatory to ensure the compatibility with ROS By Example vol1. source code examples.
It is smart to use the Command Velocity Multiplexer of the TurtleBot2, instead of publishing directly to
"mobile_base/commands/velocity"

1- Need to update rplidar-turtlebot2/turtlebot_le2i/launch/remap_rplidar_minimal.launch:
change : remap from="mobile_base/commands/velocity" to="cmd_vel"
to : remap from="cmd_vel_mux/input/navi" to="cmd_vel"

2 - Add the associated informations to readme.md