To cite pbkrtest in publications use:

  Ulrich Halekoh, Søren Højsgaard (2014). A Kenward-Roger Approximation
  and Parametric Bootstrap Methods for Tests in Linear Mixed Models -
  The R Package pbkrtest. Journal of Statistical Software, 59(9), 1-30.
  URL https://www.jstatsoft.org/v59/i09/.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {A Kenward-Roger Approximation and Parametric Bootstrap Methods for Tests in Linear Mixed Models -- The {R} Package {pbkrtest}},
    author = {Ulrich Halekoh and Søren Højsgaard},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {59},
    number = {9},
    pages = {1--30},
    url = {https://www.jstatsoft.org/v59/i09/},
  }