Add references in onboarding/packaging_guide.md

[jangorecki]

Hi,
I’m reading the doc, and among multiple references present in the doc I actually cannot find the most canonical resource on packaging matter. I think it’s worth to add https://cran.r-project.org/doc/manuals/r-release/R-exts.html at least in “Further guidance” section.
Jan

[noamross]

Thanks! Good idea, I just added it: https://github.com/ropensci/onboarding/commit/62630783c0fd5042285e0f55ee143e8815828ce3