For the near future, the documentation for these packages will be found on the GHC Dev Wiki.
https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker/RowTypes/Coxswain
Matthew Pickering wrote up a Nix config, if that's your style. I wouldn't ask him to expend energy keeping this up-to-date --- and I don't yet know how --- but I anticipate that most those of you interested in such things will probably know at least how to tweak this file as useful template.