Tutorial Covering How to Add Custom Data to Affect Routes

[kevinkreiser]

A number of potential users of valhalla want to add their own open or proprietary data to valhalla route tiles so that they can influence the route computation in certain ways. It would be nice to give a brief overview of where one might make such changes to accomplish this.

I've got a start on this already but I need to turn it into a proper tutorial. After that, it would be nice to have @rmglennon look it over to make sure it makes sense, is properly organized, reads properly etc.