Added mailing list link to README.

README

@@ -293,3 +293,8 @@ Asynchronous fork of https://github.com/wg/epgsql
       Connection  - connection the notice occurred on
       Error       - an #error{} record, see pgsql.hrl
 
+
+* Mailing list
+
+  https://groups.google.com/forum/#!forum/epgsql
+