Add comment about maildir_very_dirty_syncs to dovecot.conf

data/conf/dovecot/dovecot.conf

@@ -24,6 +24,10 @@ mail_plugins = </etc/dovecot/mail_plugins
 mail_attachment_fs = crypt:set_prefix=mail_crypt_global:posix:
 mail_attachment_dir = /var/attachments
 mail_attachment_min_size = 128k
+# Significantly speeds up very large mailboxes, but is only safe to enable if
+# you do not manually modify the files in the `cur` directories in 
+# mailcowdockerized_vmail-vol-1. Disabled by default.
+#maildir_very_dirty_syncs=yes
 
 # Dovecot 2.2
 #ssl_protocols = !SSLv3