Documentation for how to create new flexinode types/contribs/plugins?

I would like to create some custom field types for use in flexinode, but I can't seem to find any documentation on it, and the task seems rather daunting. Is there any documentation available for how to properly do this?

I imagine I could slowly try to inch my way through the code to figure it out, but my brain is old and crusty, and it'll take forever. If there are some existing docs, or hints, or anything, that would be fantastic. I tried doing a search here - but after fishing through the huge list of returned results for anything that includes "flexinode" I couldn't find any links to documentation.

Does anything like this exist?