Mention preserveFixedJoint option in ros_urdf tutorial

[osrf-migration]

Original report (archived issue) by Silvio Traversaro (Bitbucket: traversaro).

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The preserveFixedJoint option of the URDF --> SDF converter (added in https://bitbucket.org/osrf/sdformat/pull-requests/352/add-preservefixedjoint-option-to-the-urdf/diff ) is useful to make sure that URDF fixed joints are transformed to SDF fixed joints (it can be used for example to model six axis FT sensors as documented in http://gazebosim.org/tutorials?tut=force_torque_sensor&cat=sensors).

It would be useful to document this option in the ros_urdf tutorial, to reduce confusion for users (see http://answers.gazebosim.org/question/21118/converting-urdf-to-sdf-preservefixedjoint-not-working/).

A tricky aspect is to clearly document on which versions of Gazebo this option can be used.
preserveFixedJoint was added in some minor release of SDFormat 4 so it should be available using Gazebo7 (see http://gazebosim.org/tutorials?tut=install_dependencies_from_source), that in turn is compatible with ROS >= Kinetic (see http://gazebosim.org/tutorials/?tut=ros_wrapper_versions).

However, if they use Gazebo7 from the official ubuntu repositories of Ubuntu 16.04, note that no new minor release of SDFormat 4 was released for it, so you still cannot use preserveFixedJoint.