Docs for the r-base image

This PR provides our proposed documentation of the r-base image (recently merged in https://github.com/docker-library/official-images/pull/310), following the template of the other docs pages.  Looking forward to your feedback.  Thanks!
Thanks, this has been fixed (actually as that the example runs a script and not an R command it should be simply `CMD ["Rscript", "myscript.R"]`)