Link or include important discussions in design documents

[awvwgk]

@Beliavsky wrote in https://fortran-lang.discourse.group/t/2338/5:

A fifth part could be added: discussions . Most code in stdlib has one or more related issues where it has been discussed. If related discussions were listed for each source file, that would help to fill in the reference information, since people have often provided links to references when discussing algorithms. Reading discussions could also tell users why algorithms were chosen and what the alternatives are.

@certik wrote in https://fortran-lang.discourse.group/t/2338/7:

Excellent idea. Indeed, we should link all discussions for each module in stdlib. I think the discussions are usually related to the whole module, not just a single file.

It’s the same idea behind the GitHub - j3-fortran/fortran_proposals: Proposals for the Fortran Standard Committee repository: to be able to track discussions behind each Fortran feature as it is added to the standard, and features that didn’t make it (and why).

We should include or reference important issues from the design documents or even the reference as appropriate. Let's use this issue to collect important issues and discussions from the fpm repository which should be mentioned in the fpm design documents.