Aggregate Reasons for Reviewer addition

[edwardtew]

When a user is added to the PR as a Reviewer it would be helpful if there was a comment that state the reason the user had been added.

I experimented with adding a comment at the time the user was added but it resulted in a potential swamp of comments.
e.g.

Ideally the files associated with the change would be aggregated into one comment or one comment per user and then added so that the PR is not overly cluttered.