Provide tutorial showing how to add another analysis task

[timbeccue]

While the code should be fairly straightforward to follow, it's always nice to have well documented tutorials. Include one here, either in the readme or as a separate document, clearly illustrating how a new analysis task can be added. The goal is to make it as simple as possible for people who may be familiar with python but not so much with this app in particular, Github conventions, or Photon Ranch software in general.