link to pandoc is dead in README.md

[PietrH]

The link in the README which refers to https://github.com/rstudio/rmarkdown/blob/master/PANDOC.md is currently dead.

It could probably be changed to https://github.com/jgm/pandoc/blob/master/README.md

[jdblischak]

@PietrH Thanks so much for reporting the dead link. The file I was pointing to was moved in rstudio/rmarkdown#1574. I prefer to link to the RStudio-provided instructions for installing pandoc because they detail the requirements and options for using pandoc with the rmarkdown package.

[PietrH]

@jdblischak No problem, there's still the link to pandoc.com for more general information. Thank you for the quick fix!