To cite the USAboundaries package in publications, please cite the
paper in the Journal of Open Source Software:

  Lincoln A. Mullen and Jordan Bratt, "USAboundaries: Historical and
  Contemporary Boundaries of the United States of America," Journal of
  Open Source Software 3, no. 23 (2018): 314,
  https://doi.org/10.21105/joss.00314.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{USAboundaries}: Historical and Contemporary Boundaries
of the United States of America},
    author = {Lincoln A. Mullen and Jordan Bratt},
    journal = {Journal of Open Source Software},
    year = {2018},
    volume = {3},
    issue = {23},
    pages = {314},
    url = {https://doi.org/10.21105/joss.00314},
    doi = {10.21105/joss.00314},
  }