To cite 'posterdown' in publications use:

Thorne WB (2019). posterdown: An R Package Built to Generate Reproducible Conference Posters for the Academic and Professional World Where Powerpoint and Pages Just Won't Cut It. R package version 1.0, https://github.com/brentthorne/posterdown.

Corresponding BibTeX entry:

  @Manual{,
    title = {posterdown: An R Package Built to Generate Reproducible
      Conference Posters for the Academic and Professional World Where
      Powerpoint and Pages Just Won't Cut It},
    author = {W. Brent Thorne},
    year = {2019},
    url = {https://github.com/brentthorne/posterdown},
    note = {R package version 1.0},
  }