minor README update

README

@@ -9,9 +9,10 @@ communicate desired access to a service that is hidden behind a firewall in a
 default-drop filtering stance. The main application of SPA is to use a firewall
 to drop all attempts to connect to services such as SSH in order to make the
 exploitation of vulnerabilities (both 0-day and unpatched code) more difficult.
-Any service that is concealed by SPA naturally cannot be scanned for with Nmap.
-The fwknop project supports three different firewalls: iptables on Linux
-systems, pf on OpenBSD, and ipfw on FreeBSD and Mac OS X.
+Because there are no open ports, any service that is concealed by SPA naturally
+cannot be scanned for with Nmap.  The fwknop project supports three different
+firewalls: iptables on Linux systems, pf on OpenBSD, and ipfw on FreeBSD and
+Mac OS X.
 
 SPA is essentially next generation Port Knocking (PK), but solves many of the
 limitations exhibited by PK while retaining its core benefits. PK limitations