A spec. There is no substitute for it. Code isn't a spec. A spec is documentation that describes a format and/or operation. The absence of it is laziness and/or arrogance.
On the other hand - it's an open source project, I hope someone can contribute to it and create a PR with the spec.
There are some unfortunately popular projects like Puppet over the past 20 years who tried to sell the lack of documentation as a positive, or just had incomplete and outdated documentation like Chef. ;] On the flip side, IETF RFCs, Python PEPs, and Rust RFCs are examples where specifications made things clearer and open to everyone. It's not enough to open source a thing without context or comments, the learning curve must be accounted for to make it as self-service and understandable as possible to people who have domain knowledge but have never used this particular thing before. OOTBE UX. Code maybe a communication of intended system behavior, but it is often too low-level and spread around to be relied upon as a singular, compact reference.