Add link to `fwknop`

The `fwknop` tool is very useful to protect services on servers via
single packet authorization.

README.md

@@ -442,6 +442,7 @@ Comparison of NoSQL servers: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
 * [Blackbox](https://github.com/StackExchange/blackbox) - Safely store secrets in Git/Mercurial. Provides tooling to automatically encrypt secrets like passwords.
 * [Denyhosts](http://denyhosts.sourceforge.net/) - Thwart SSH dictionary based attacks and brute force attacks.
 * [Fail2Ban](http://www.fail2ban.org/wiki/index.php/Main_Page) - Scans log files and takes action on IPs that show malicious behavior.
+* [fwknop](https://www.cipherdyne.org/fwknop/) - Protects ports via Single Packet Authorization in your firewall.
 * [Glastopf](http://glastopf.org/) - A low-interaction web application honeypot to emulate vulnerabilities and gather attack data.
 * [Kippo](https://github.com/desaster/kippo) - A medium-interaction SSH honeypot, mostly used as a standalone SSH daemon with a configurable Filesystem sandbox.
 * [OSSEC](http://ossec.net) - OSSEC is a HIDS that performs log analysis, FIM, rootkit detection, and much more.