To cite rtweet use:

  Kearney, M. W. (2019). rtweet: Collecting and analyzing Twitter data,
  Journal of Open Source Software, 4, 42. 1829. doi:10.21105/joss.01829
  (R package version 0.7.0)

A BibTeX entry for LaTeX users is

  @Article{rtweet-package,
    title = {rtweet: Collecting and analyzing Twitter data},
    author = {Michael W. Kearney},
    year = {2019},
    note = {R package version 0.7.0},
    journal = {Journal of Open Source Software},
    volume = {4},
    number = {42},
    pages = {1829},
    doi = {10.21105/joss.01829},
    url = {https://joss.theoj.org/papers/10.21105/joss.01829},
  }