add set fingerprints example

ok deraadt@ henning@ frantzen@

src/etc/pf.conf

@@ -1,4 +1,4 @@
-#	$OpenBSD: pf.conf,v 1.20 2003/06/17 21:48:10 david Exp $
+#	$OpenBSD: pf.conf,v 1.21 2003/09/02 20:38:44 david Exp $
 #
 # See pf.conf(5) and /usr/share/pf for syntax and examples.
 # Required order: options, normalization, queueing, translation, filtering.
@@ -27,6 +27,7 @@
 #set optimization normal
 #set block-policy drop
 #set require-order yes
+#set fingerprints "/etc/pf.os"
 
 # Normalization: reassemble fragments and resolve or reduce traffic ambiguities.
 #scrub in all