Update our policies on relevancy

[avivkeller]

Given that our tooling is going to be more widely adopted, I propose we implement a new policy limiting 1 change per 1 PR. That way, in the event of a break, we have easier ability to isolate changes, and it'll be much easier for reviewers to understand changes.

[bmuenzenmeyer]

do you feel this needs to be in writing? I suppose its a bit of an unwritten norm. Just curious.
when in doubt, having a url that we can use to enforce our rules is a good thing