The package is still in development, although different people have already made good use of it. This means that breaking changes might still happen more than you would like. Once we are close to a first major release (i.e. a first version on CRAN hopefully), this will of course not be the case anymore. A good place to start is the vignette “A brief overview”.
Using R and MODFLOW, and wanting to contribute? Please do so! The most straightforward contributions at the moment are in the form of file reading, writing and object creation functions for the many still-not-supported MODFLOW packages. I intend to create a vignette for contributors, listing what packages we are currently supporting etc., but it is not there yet. It is probably not too hard to find out yourself however. Just make sure you do things the way we do, and you follow the online guide to MODFLOW meticulously (note we use lowercase versions of all parameters here). A good starting point is probably to have a close look at
rmf_read_dis() and the corresponding page in the online guide. Note also that we are first focussing on MODFLOW-2005 packages. Later on the other derived versions of MODFLOW-2005 might be supported as well. MODFLOW-2000 and MODFLOW 6 are out of scope.
There are many ideas for further development. Here are a few to give you a taste of where this is going: