Adds and updates urdf ecosystem diagram (#33)

- Migrates urdf_diagram.odg and urdf_diagram.png from robot_model pkg
to this urdf pkg

- Updates urdf_diagram for current repo structures and urls

- Adds rendering from diagram in README.md

Author: IanTheEngineer

README.md

@@ -6,5 +6,8 @@ See the ROS wiki for API documentation and tutorials.
 * [`urdf`](http://wiki.ros.org/urdf)
 * [`urdf_parser_plugin`](http://wiki.ros.org/urdf_parser_plugin)
 
-This was originally part of the [`ros/robot_model`](https://github.com/ros/robot_model) repository.
-It has been moved to this repo as described by [`ros/robot_model#195`](https://github.com/ros/robot_model/issues/195)
+# URDF Ecosystem
+![urdf_diagram](urdf/documentation/urdf_diagram.png)
+
+These packages were originally part of the [`ros/robot_model`](https://github.com/ros/robot_model) repository.
+They have been moved to this repo as described by [`ros/robot_model#195`](https://github.com/ros/robot_model/issues/195)