Documentation for contributors

[noamross]

A couple of thoughts on helpful documentation for contributors:

1. A pithy "R-Multiverse packages must meet the following requirements to be in the community repo:", and "In addition, they must meet the following to be in the production repo:" would be helpful for contributors and for users or other stakeholders to understand what the QA standards are.

2. A short section documenting how packages can depend on, or be dependencies of, CRAN packages and what authors should do to best enable this would be helpful. This would be applicable to R-Universe generally and we could add it to our docs, but with documentation I think repetition is helpful. (At RO we often abide by the "principle of multiple points of entry," meaning that you should assume a user may stumble upon documentation from any referral and not always find the canonical source).

[wlandau]

Promoting this discussion to an issue: #105