doc: add VERSION section to manpages

This shall make it easier to identify outdated HTML renditions on the
interwebs, since many of them do not display the .TH header like man(1)
does.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/iptables.8.in b/iptables.8.in
index d29deb2..f36d220 100644
--- a/iptables.8.in
+++ b/iptables.8.in
@@ -427,3 +427,6 @@
 .\" .. sexy, too ..
 .\" .. witty, charming, powerful ..
 .\" .. and most of all, modest ..
+.SH VERSION
+.PP
+This manual page applies to iptables @PACKAGE_VERSION@.