To cite package ‘checkdown’ in publications use:

  Moroz G (2020). _Create check-fields and check-boxes with checkdown_.
  <https://CRAN.R-project.org/package=checkdown>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {Create check-fields and check-boxes with checkdown},
    author = {George Moroz},
    year = {2020},
    url = {https://CRAN.R-project.org/package=checkdown},
  }