README: Add mailing list

README.md

@@ -17,6 +17,8 @@ The project page (issue, wiki, etc) is here:
 
   https://code.ffdn.org/FFDN/coin/
 
+A mailing list is available, for both users and developers: https://listes.illyse.org/wws/info/si
+
 
 Demo
 ====