Is there documentation on the structure of the TF code base?

I’m starting to familiarize myself with TF internals so that I can contribute. Is there any developer documentation that covers things like code base structure, common conventions/macros/internal classes, etc.? The docs that I’ve seen so far are geared more toward end users who are calling the API, rather than for contributors to TF itself.

Lack of documentation won’t deter me from continuing to learn, but I would love to use any resources that are available.