[R|system] [package|library] discipline

[jennybc]

In reading https://github.com/r-windows/docs/blob/master/rtools40.md, it strikes me there's a lot of potential for people to confuse themselves (or misinterpret what's written) around all possible combinations of "R | system" and "package | library". Many R users are pretty foggy on the difference between an R package and an R library, to begin with. And this document really requires clear thinking about all of these concepts.

I propose a mini glossary near the top explaining the terms that will be used and exactly what they mean, with an example.