Add Citations to the RMarkdown Document/Report and Print Bibliography

[ratral]

Information:
https://bookdown.org/yihui/bookdown/citations.html
https://de.sharelatex.com/learn/Bibliography_management_with_bibtex
https://rmarkdown.rstudio.com/authoring_bibliographies_and_citations.html
https://www.r-bloggers.com/writing-manuscripts-in-rstudio-easy-citations/amp/