Add action to render README.Rmd when it changes

[cjyetman]

For R packages that use a Rmd README, if often happens that the Rmd is modified, but not rendered. We have an action that does this, but I'm not sure if it works perfectly, e.g. https://github.com/RMI-PACTA/pacta.portfolio.allocate/blob/main/.github/workflows/render-readme.yml

[jdhoffa]

It doesn't seem to work at all, I have been trying to debug it today actually :-(

Some weirdness with renv and lockfiles and a bunch of crap like that.