add an urdf model example

[osrf-migration]

Original report (archived issue) by John Hsu (Bitbucket: hsu, GitHub: hsu).

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Currently it looks like all models have been converted to sdf.
We should support URDF format for ROS community.

[osrf-migration]

Original comment by Nate Koenig (Bitbucket: Nathan Koenig).

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There is an sdf_to_urdf branch in sdformat that will be the way ROS users can make use of SDF files.