Clarify that r? works in comments.

Author: iwanders

src/contributing.md

@@ -77,7 +77,7 @@ All pull requests are reviewed by another person. We have a bot,
 to review your request based on which files you changed.
 
 If you want to request that a specific person reviews your pull request, you
-can add an `r?` to the pull request description. For example,
+can add an `r?` to the pull request description or comment. For example,
 [Steve][steveklabnik] usually reviews documentation changes. So if you were to
 make a documentation change, add
 
@@ -95,6 +95,7 @@ team by adding:
 For a full list of possible `groupname` check the `groups` section at the
 [rust highfive config file](https://github.com/rust-lang/highfive/blob/master/highfive/configs/rust-lang/rust.json).
 
+
 ### CI
 
 In addition to being reviewed by a human, pull requests are automatically tested