Add EXAMPLES-section entry for new key+=append syntax of sysrc(8).

MFC after:	3 days
X-MFC-to:	stable/10 stable/9
X-MFC-with:	274068
Thanks to:	@SeanChittenden

usr.sbin/sysrc/sysrc.8

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd November 3, 2014
+.Dd November 4, 2014
 .Dt SYSRC 8
 .Os
 .Sh NAME
@@ -247,6 +247,12 @@ Working on other files, such as
 -f /etc/crontab MAILTO
 .Dl returns the value of the MAILTO setting Pq if configured .
 .Pp
+Appending to existing values:
+.Pp
+.Nm
+\&cloned_interfaces+=" gif0"
+.Dl appends Qo \ gif0 Qc to $cloned_interfaces .
+.Pp
 In addition to the above syntax,
 .Nm
 also supports inline