To cite HLMdiag in publications use:

  Adam Loy, Heike Hofmann (2014). HLMdiag: A Suite of Diagnostics for
  Hierarchical Linear Models in R. Journal of Statistical Software,
  56(5), 1-28. URL https://www.jstatsoft.org/article/view/v056i05.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{HLMdiag}: A Suite of Diagnostics for Hierarchical Linear Models in {R}},
    author = {Adam Loy and Heike Hofmann},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {56},
    number = {5},
    pages = {1--28},
    url = {https://www.jstatsoft.org/article/view/v056i05},
  }