This is organized more like a FAQ than a guide, but it gives all the information you probably need to hack on the Mongrel2 source code. It talks about getting the code, doing your first patch, coding guidelines, etc. These aren't meant to be strict rules but more guidelines to follow.
Mongrel2 uses a BSD 3-clause license which should meet most people's needs.
The idea for Mongrel2 is slightly controversial, but it has potential. Here's some criticisms about the proposed design, and I'm looking for more.
Getting Started With Mongrel2Dec-21-2021
This is the fastest way to get started with Mongrel2. Nothing is really explained, just all the stuff you need is setup and you get to run a few commands. If you want very good explanations for all of this, go read The Mongrel2 Manual a complete manual covering everything from getting started, to writing your first handlers.