update CONTRIBUTING doc

Tell to open a new ticket instead of commenting an older one. It makes
difficult to track and apparently notifications are not received.

CONTRIBUTING.md

@@ -43,8 +43,12 @@ documenting your bug report or improvement proposal. If it does, it
 never hurts to add a quick "+1" or "I have this problem too". This will
 help prioritize the most common problems and requests.
 
+
 ### Conventions
 
+Don't comment on closed issues or PR, instead open a new issue and link it to
+the old one.
+
 Fork the repo and make changes on your fork in a feature branch:
 
 - If it's a bugfix branch, name it XXX-something where XXX is the number