To cite countrycode in publications use:

  Arel-Bundock V, Enevoldsen N, Yetman C (2018). “countrycode: An R
  package to convert country names and country codes.” _Journal of Open
  Source Software_, *3*(28), 848.
  <https://doi.org/10.21105/joss.00848>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {countrycode: An R package to convert country names and country codes},
    author = {Vincent Arel-Bundock and Nils Enevoldsen and CJ Yetman},
    journal = {Journal of Open Source Software},
    year = {2018},
    volume = {3},
    number = {28},
    pages = {848},
    url = {https://doi.org/10.21105/joss.00848},
  }