Advise reader to also see mdconfig(8) in mount_cd9660(8).

It's useful for how to mount an iso file via loopback.

Reviewed by:	jilles
Approved by:	bcr (mentor)
Differential Revision:	https://reviews.freebsd.org/D16067

sbin/mount_cd9660/mount_cd9660.8

@@ -32,7 +32,7 @@
 .\"     @(#)mount_cd9660.8	8.3 (Berkeley) 3/27/94
 .\" $FreeBSD$
 .\"
-.Dd March 22, 2017
+.Dd August 11, 2018
 .Dt MOUNT_CD9660 8
 .Os
 .Sh NAME
@@ -140,6 +140,7 @@ The following command can be used to mount a Kodak Photo-CD:
 .Xr unmount 2 ,
 .Xr cd9660 5 ,
 .Xr fstab 5 ,
+.Xr mdconfig 8 ,
 .Xr mount 8
 .Sh HISTORY
 The