fixup! mention if others touch the same branch

Author: jbergstroem

COLLABORATOR_GUIDE.md

@@ -220,5 +220,6 @@ With git, there's a way to override remote trees by force pushing
 you're rewriting history on a repository other people are working
 against) but is allowed for simpler slip-ups such as typos in commit
 messages. You are only allowed to force push to any io.js branch
-within 10 minutes from your original push. After that period passes,
+within 10 minutes from your original push. If someone else pushes to
+the branch your commit lives in or the 10 minute period passes,
 consider the commit final.