doc: document R CRAN mirror process

The current benchmark docs showing how to install R packages
doesn't go into how to specify a CRAN mirror.

This commit shows how to specify the repo parameter when
installing a package.

Fixes: https://github.com/nodejs/node/issues/10204
PR-URL: https://github.com/nodejs/node/pull/10211


Reviewed-By: Andreas Madsen <amwebdk@gmail.com>