Expand CONTRIBUTING on how to make things easy for reviewers

[annevk]

https://github.com/whatwg/meta/blob/master/CONTRIBUTING.md#pull-requests should say something about how to make it easier for reviewers to follow what is happening. This is what I wrote elsewhere on that topic:

For ease of review I'd greatly appreciate two things: 1) Resolve addressed feedback so it's clear what is considered done (the reviewer can reopen if there's disagreement). I've taken the liberty of resolving most. 2) Use fixup commits as these make it very clear to the reviewer how a set of comments got resolved. We can always squash near the end. If rebasing is needed due to conflicts I would recommend rebasing first and then addressing feedback on top as subsequent commits.