Add detail to contributing guide

Author: apiraino

src/contributing.md

@@ -96,6 +96,18 @@ For a full list of possible `groupname` check the `adhoc_groups` section at the
 or the list of teams in the [rust-lang teams
 database](https://github.com/rust-lang/team/tree/master/teams).
 
+---
+
+**Please notice**: we receive way more pull requests than we have reviewers available, many of them are contributing on a volunteer basis. In order to minimize review delays, pull request authors and assigned reviewers should ensure that the review label (`S-waiting-on-review` and `S-waiting-on-author`) stays updated, invoking these commands when appropriate:
+
+`@rustbot author`: the review is finished, PR author should check the comments and take
+action accordingly
+
+`@rustbot review`: the author is ready for a review, this PR will be queued again in the
+reviewer's queue
+
+---
+
 ### CI
 
 In addition to being reviewed by a human, pull requests are automatically tested