Add a package design vignette

[pratikunterwegs]

This issue is to request the addition of a package design document as a vignette. This will serve as a trial and potential template for additional design documents for other packages.

[avallecam]

Is there space for some personas as target users' profiles? I'd like to test if may be helpful or informative for defining or delimiting some negotiable or non-negotiable package design principles. If not for this package, possibly for the next one.

[pratikunterwegs]

Thanks @avallecam - we do have a section in the draft template for the use case, which includes a brief description of the target users. We could add a note that package authors could reference some of the personas if they feel they are useful. Ideally though, the documents would be stand-alone, so not requiring much reference externally.