ReviewingGuidelines: encourage positive reviews more

ReviewingGuidelines: encourage positive reviews more

I saw some contributors hesitate to give a positive review on
patches by their coworkers.  When written well, a positive review
does not have to be a hollow "looks good" that rubber stamps an
useless approval on a topic that is not interesting to others.

Let's add a few paragraphs to encourage positive reviews, which is a
bit harder to give than a review to point out things to improve.

Signed-off-by: Junio C Hamano <gitster@pobox.com>