doc: add section stating that very stale PRs should be closed

doc/contributing/pull-requests.md

@@ -446,6 +446,11 @@ credit for the work they started (either by preserving their name and email
 address) in the commit log, or by using an `Author:` meta-data tag in the
 commit.
 
+If a pull request has been inactive for more than six months, add the `stalled` label
+to it. That will trigger an automation that adds a comment explaining the pull request
+may be close for inactivity, giving a heads-up to the contributor before actually
+closing it if it remains inactive.
+
 ### Approving a change
 
 Any Node.js core collaborator (any GitHub user with commit rights in the