To cite package ‘maddison’ in publications use:

  Persson E (2019). _maddison: Maddison Project Database_. R package
  version 0.2, <https://github.com/expersso/maddison>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {maddison: Maddison Project Database},
    author = {Eric Persson},
    year = {2019},
    note = {R package version 0.2},
    url = {https://github.com/expersso/maddison},
  }