Document coding style used by R Core

[SaranjeetKaur]

Issue related to the R Dev Guide: r-devel/rdevguide#133.

Info on how to contribute to the R Dev Guide are available in the Contributing Guidelines.

[hturner]

It would be good to pick this up again at RSECon25. r-devel/rdevguide#216 addressed the initial checklist in r-devel/rdevguide#133 (comment) but did not address the points in r-devel/rdevguide#133 (comment), which require review of some references to pull out items to add to the R Dev Guide.