Discourage use of flat file stats logging.

- Current XML-based file formatting performs very poorly; we should investigate a faster alternative (like an ongoing CSV write) if we want a production-friendly file writer.

config/vufind/config.ini

@@ -382,7 +382,8 @@ ils_encryption_key = false
 ; "stats" core here; if no URL is specified, [Index]/url below will be used.
 ;solr            = http://localhost:8080/solr
 
-; When using the File mode, specify a directory for saving stat files here:
+; When using the File mode, specify a directory for saving stat files here.
+; NOTE: As currently implemented, File mode is NOT RECOMMENDED FOR USE IN PRODUCTION.
 ;file            = /usr/local/vufind/local/logs
 
 ; When displaying search statistics in the Admin module, do you want to see a single