R addons have a system of dependencies that is similar to Fedora: hard dependencies (LinkingTo, Depends, Imports), and soft dependencies (Suggests) that are not required to run the package. Historically, we added everything to BuildRequires, which creates very nasty circular loops that are very hard to disentangle when a mass rebuild is required (e.g. for a new release of R).

I suppose that we have been doing this because many Suggests are used for building vignettes and other pieces of documentation, and these are rebuilt by default as part of the checks, which is pointless. For some time now, I've been trying to disentangle this mess, dropping Suggests and adapting the checks, but we need to add this to the guidelines too.

A PR follows adapting the guidelines to current practices regarding the discussion above as well as other minor bits.