To cite precrec in publications use:

  Takaya Saito and Marc Rehmsmeier (2017). Precrec: fast and accurate
  precision-recall and ROC curve calculations in R. Bioinformatics
  (2017) 33 (1): 145-147. https://doi.org/10.1093/bioinformatics/btw570

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Precrec: fast and accurate precision-recall and ROC curve calculations in R},
    author = {Takaya Saito and Marc Rehmsmeier},
    journal = {Bioinformatics},
    year = {2017},
    volume = {33 (1)},
    pages = {145-147},
    doi = {10.1093/bioinformatics/btw570},
  }