Submit pull request reviews using GitHub's "Review changes" button

[cbeams]

When reviewing a pull request, do not just add a normal comment, but actually click the "Review changes" button and add your summary using the ACK / NACK language at #6.

This makes it easier for everyone to see that a review has already been submitted for a given pull request without having to scroll through comments, makes it possible to search for pull requests without reviews, pull requests reviewed by a certain user, etc. It also makes the review comment itself that much more prominent, because it shows up with a big green checkmark, red X.