Adding comments on when to use each review type (#661)

* Adding comments on when to use each review type

Fixes https://github.com/gomods/athens/issues/545

* Adding comment about reviewing 'comment' PRs

REVIEWS.md

@@ -25,4 +25,54 @@ Here they are:
     review, so keep the pull request open for 24-36 hours before merging.
 - Pull requests have to pass continuous integration (CI) tests.
 
+# Review Types and When to Use Them
 
+We use the [Github Pull Request Review](https://help.github.com/articles/about-pull-request-reviews/)
+system to review pull requests. Overall, we think it's a pretty intuitive system
+with a nice user interface. Hopefully you'll get the hang of it pretty quickly. If you
+don't, no worries - ask us in the `#athens` room on the [Gophers Slack](https://invite.slack.golangbridge.org/)!
+
+When you're doing a review on a PR, you'll make comments that nobody but you can see until you
+submit the review. That feature can be nice because you might want to change things as you
+learn more about the code, etc... 
+
+It can also be confusing if you forget to submit your review! Lots of us have forgotten to do 
+that :smile:.
+
+Anyway, when you're done with your review and satisfied with all your comments, you'll have 
+three options to submit your review:
+
+- Comment
+- Approve
+- Request Changes
+
+Below, we'll explain when to use each of these options.
+
+## Request Changes
+
+When you decide to submit your review, please use `Request Changes` when you've posted
+comments asking the author to make some changes, explain something, etc..., and you 
+want to block the PR from being merged until:
+
+- The author has a chance to read your comments and make changes, etc...
+- You have a chance to review again
+- You explicitly approve in your review
+
+## Comment
+
+When you decide to submit your review, please use `Comment` for feedback that you'd
+like addressed but it's not crucial if it's not. This review type will allow another maintainer
+to merge the PR after your feedback has been addressed, without requiring you to 
+come back and manually re-review and approve the PR. In other words, you won't
+be blocking the PR from being merged.
+
+Conversely, if you are considering whether to merge a PR, please make sure that other 
+reviewers' `Comment` reviews are addressed (e.g. the author has made requested changes, 
+responded to questions, etc...) before you merge.
+
+## Approve
+
+This one is the easiest. When you decide to submit your review, please use `Approve` if everything
+looks good. If nobody else has any `Request Changes` or `Comment` reviews 
+(GitHub will show a red "X" near their name if they do), you can click the "Squash and Merge"
+button to merge their PR into the master branch!