To cite brokenstick in publications use:

van Buuren S (In preparation). “Broken Stick Model for Irregular Longitudinal Data.” https://github.com/growthcharts/brokenstick.

Corresponding BibTeX entry:

  @Misc{,
    title = {Broken Stick Model for Irregular Longitudinal Data},
    author = {Stef {van Buuren}},
    journal = {Journal of Statistical Software},
    year = {In preparation},
    url = {https://github.com/growthcharts/brokenstick},
  }