Write a tutorial on using ignition fuel models

[bsb808]

Maybe this already exists externally, but now that we have our DAVE collection, it would be cool to have a wiki tutorial illustrating how to use the models from the collection.

[mabelzhang]

We have a pretty comprehensive tutorial on the Ignition website here https://ignitionrobotics.org/api/gazebo/6.5/meshtofuel.html

The wiki page should at a minimum link to that, as it gives information about how the model directory should be structured, which is useful for uploading models. It also gives an example of how to use the model in a world file - it's literally a one-step process of copy-pasting the URI on the webpage and throwing it into the SDF <include><uri>. This process is the same for both Gazebo and Ignition, as it's an SDF feature, agnostic to the simulator.