Fix references

[krpaulson]

So far, I tried using this as a guideline: https://cran.r-project.org/web/packages/Rdpack/vignettes/Inserting_bibtex_references.pdf

The steps I took were:
(1) Add to DESCRIPTION
Imports: Rdpack
RdMacros: Rdpack
(2) Add to globals file to add to NAMESPACE:
#' @importFrom Rdpack reprompt
(3) Create inst/REFERENCES.bib file with sample references in .bib format
(4) Add to @references section in roxygen headers:
\insertRef{key}{lifetableUtils}

When I did this the references did not render correctly. Need to revisit this.

[krpaulson]

From @spencerpease : "If getting in-documentation citations to work is difficult, maybe we could make a citations() function or vignette that just lists these citations (and maybe how to cite this package if someone (hopefully) wanted to use it)."

[spencerpease]

I just came across some information on using citations in R Markdown documents in the RStudio cheatsheet, on the second page. It might help with at least adding citations to the vignettes.