In my markdown documents, I use the following language to describe versions:
# Software versions and time stamps Package `MCMCglmm` version `r packageVersion("MCMCglmm")` was used to fit Bayesian models. Analysis has been performed under R version `r getRversion()`. This document was compiled using `RMarkdown` version `r packageVersion("RMarkdown")` and `knittr` version `r packageVersion("knitr")` on `r Sys.time()`.
For the benefit of others, I think it might be helpful to have something like that in the template that RStudio throws at you when you File/New File/R Markdown. I am sure it will be more commonly used than
r summary(cars) and
r plot(pressure) ;).