nav2_msgs readme (#257)

* nav2_msgs readme

* adding readme nav2_msgs suggestions

* removing detailed walk through for link to ROS 2 Migration Guide

nav2_msgs/README.md

+# nav2_msgs
+
+The `nav2_msgs` package is a set of messages, services, and actions for the `navigation2` stack. The `navigation2` stack still makes use of `nav_msgs` from ROS (1) Navigation.
+
+See the ROS 1 to ROS 2 [Migration Guide](https://index.ros.org/doc/ros2/Migration-Guide/#messages-and-services) for details about use of the new message and service types.