I don’t believe we have a guide for adding them specifically to the freedom platform but there have been guides for adding custom instructions via the RoCC interface to rocket-chip in the past see a brief introducton and a VERY old tutorial. All the documents I know have become out of date as rocket-chip has continued to evolve so I think they would only serve as background on the interface but not a getting started in the current code base.
If you wanted to look at the code base as it stands now there are several simple examples in rocket-chip of RoCC accelerators e.g. here
Hopefully this will help you get started.