add reference to the ratelimit logs

service/crm.md

@@ -365,6 +365,9 @@ to syslog on `crm-int-01`. Those logs can be read using `journalctl -t
 processor`. They can be useful to determine the cause of donations being
 submitted but not showing up in CiviCRM.
 
+The PHP frontend *should* also be logging. It logs credit card errors
+to `/srv/donate.torproject.org/htdocs-prod/logs/ratelimit.log`
+
 ## Backups
 
 Backups are done with the regular [backup procedures](howto/backup) except for