Update default config with comments

conf/app.ini

@@ -181,9 +181,14 @@ SERVICE = server
 DISABLE_GRAVATAR = false
 
 [attachment]
+; Path for attachments. Defaults to files/attachments
 PATH = 
 ; One or more allowed types, e.g. image/jpeg|image/png
 ALLOWED_TYPES = 
+; Max size of each file. Defaults to 32MB
+MAX_SIZE
+; Max number of files per upload. Defaults to 10
+MAX_FILES =
 
 [log]
 ROOT_PATH =