To cite qqman in publications use:

  Stephen D. Turner (2018). qqman: an R package for visualizing GWAS
  results using Q-Q and manhattan plots. Journal of Open Source
  Software, 3(25), 731 doi:10.21105/joss.00731

A BibTeX entry for LaTeX users is

  @Article{,
    title = {qqman: an R package for visualizing GWAS results using Q-Q and manhattan plots},
    author = {Stephen Turner},
    journal = {The Journal of Open Source Software},
    year = {2018},
    doi = {10.21105/joss.00731},
  }