Mention rig for managing R versions

[rig](https://github.com/r-lib/rig) has become a popular way to manage different versions of R across platforms. 

We could add this to the [R Patched and Development Versions](https://contributor.r-project.org/rdevguide/chapters/getting_started.html) as way to switch between r-release and r-devel, or between multiple development versions ("raw" and modified).