That image is quite useful, but I still think several of the modules could do with commenting in their code at least at the top which simply describes what they are supposed to do or what they're for. I mean, just looking at the repository, you'd think that Core is the only folder you need to look in and that all the others just make libraries that add on to the functionality of Core, but that's obviously (to me now) not the case. As a beginning contributor, I have little idea of what to add to and how to add it without more documentation than is available.
From my personal experience, while unit testing is an incredibly useful supplement to documentation and comments, it's not enough on its own.