Community guidelines for smaller PRs

[haydentherapper]

As discussed in this week's community meeting, there was a strong desire to see smaller PRs. This may be hard to enforce via a bot, so we should initially rely on reviewers to ask contributors to keep PRs small. Combined with #45, it should be easy for reviewers to keep track of large feature development across multiple PRs.

Context: New contributor friction log

cc @mattmoor