Dear Yorick,
any contribution is of course very welcome!
We don't have a formal developer guide yet. But this groups purpose was among other things to discuss and establish rules, naming conventions etc. Hence I suggest you browse through the discussions here and maybe you can collect/summarize relevant information: My policy so far is to encourage new contributors to contribute that what they miss most. :)
To become a contributor, you write a mail to me (for instance, by using the "Reply to Author" function) and I put you in the committer list.
In addition, it would be most welcome if you introduced yourself a bit and tell others who you are, what your experiences and interests are, etc.
Once you're are listed as committer, you'll be able to browse the change history. This will be useful in your case as I know that Daniel already did some work in the field of monad transformers. It would be good to discuss technical and design maters with him.
Apart from this, you're free to do what you think fits your interests best. Up to now we are a tiny community and the (unwritten) agreement seems to be to announce and discuss all matters here in an informal, friendly and respectful atmosphere.
Regards, Ingo