add 'waiting-for-review' incantation to main contrib page

Author: RalfJung

src/contributing.md

@@ -148,7 +148,10 @@ branch.  Also, please make sure that fixup commits are squashed into other
 related commits with meaningful commit messages.
 
 If you encounter merge conflicts, your PR will get marked as `S-waiting-on-author`.
-When you resolve them, you should use `@rustbot` to mark it as `S-waiting-on-review`.
+When you resolve them, you should use `@rustbot` to mark it as `S-waiting-on-review`:
+
+    @rustbot label -S-waiting-on-author +S-waiting-on-review
+
 See [this chapter][labeling] for more details.
 
 GitHub allows [closing issues using keywords][closing-keywords]. This feature