Commonly used messages in ROS. Includes messages for actions (actionlib_msgs), diagnostics (diagnostic_msgs), geometric primitives (geometry_msgs), robot navigation (nav_msgs), and common sensors (sensor_msgs), such as laser range finders, cameras, point clouds.
and cleanup doc a bit.
Warnings were mentioned by @mikeferguson in https://github.com/ros/common_msgs/pull/16#issuecomment-89513070