Add note to issue template

Asks people to write to the mailing list

Author: abyrd

ISSUE_TEMPLATE

@@ -1,3 +1,7 @@
+**NOTE:** this issue system is intended for reporting bugs and tracking progress in software development. For all other usage and software development questions or discussion, please write to one of the mailing lists (Google groups):
+https://groups.google.com/forum/#!forum/opentripplanner-dev
+https://groups.google.com/forum/#!forum/opentripplanner-users
+
 ## Expected behavior
 
 ## Observed behavior