To cite tufterhandout in publications use:

Sachs MC (2015). tufterhandout: Tufte-style html document format for rmarkdown. R package version 1.2.1, http://sachsmc.github.io/tufterhandout.

Corresponding BibTeX entry:

  @Manual{,
    title = {tufterhandout: Tufte-style html document format for
      rmarkdown},
    author = {Michael C. Sachs},
    year = {2015},
    note = {R package version 1.2.1},
    url = {http://sachsmc.github.io/tufterhandout},
  }