You can reference citations, too. For example, we are using the bookdown package (Xie 2020) in this sample book, which was built on top of R Markdown and knitr (Xie 2015).
Xie, Yihui. 2015. Dynamic Documents with R and Knitr. 2nd ed. Boca Raton, Florida: Chapman; Hall/CRC. http://yihui.name/knitr/.
Xie, Yihui. 2020. Bookdown: Authoring Books and Technical Documents with R Markdown. https://CRAN.R-project.org/package=bookdown.