To cite PROJ in publications use:

  PROJ contributors (2018). PROJ coordinate transformation software
  library. Open Source Geospatial Foundation. URL https://proj4.org/.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {{PROJ} coordinate transformation software library},
    author = {{PROJ contributors}},
    organization = {Open Source Geospatial Foundation},
    year = {2018},
    url = {https://proj4.org/},
  }