Add guidelines for feature requests to documentation

[wearhere]

This might take a couple of forms:

Positive guidelines (this kind of request is likely to be approved). It might be easier (better?) to define broad philosophical guidelines for the direction of the framework, than specific traits of successful PRs. Note that essential characteristics of PRs (tests & documentation) are already called out in the contributing guidelines.

Negative guidelines (this kind of request is not likely to be approved). An example of this would be ordered tests--though as with this case, we should say why we aren't likely to approve it, and hopefully provide an alternate solution (e.g. run groups). We should stress that we welcome discussion in case that alternate solution doesn't meet someone's needs.

A roadmap to show what's being worked on and thus reduce duplication.