Make a video walkthrough of the internals (with perhaps a better diagram)

[mcmire]

A couple of contributors have mentioned that they would have liked to propose some changes, except that they didn't understand the codebase. This isn't surprising, as there is quite a bit going on here. I think it would be nice to record a video that provides a high-level overview of the codebase and pulls back the curtain on how its pieces work together to produce a diff. A while back I created a diagram, but this probably needs to be updated and/or simplified to be more understandable.