add a flowchart detailing workflow

I'm working on this and would like to incorporate some feedback about decision-making from one of the end-of-course surveys. Here's what it said:

_I think it would be incredibly useful if you could add a checklist for each step, somewhat like a list of sanity checks to keep in mind at each step so you know you've considered the right assumptions for making decisions for your analysis and don't miss anything._

I like the idea of adding this into the workflow. Andreas Mueller did this for machine learning, and I found it helpful. See the following graphic.