Add checking TCP SYN cookies to recommendations docs

[zhyatt]

SYN cookies are enabled by default in Linux (https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt): Send out syncookies when the syn backlog queue of a socket overflows. This is to prevent against the common 'SYN flood attack'.

But enabling them is OS dependent. Most Linux distributivess have SYN cookies enabled by default, Windows has similar mechanism if there is high network traffic. Unfortunately it seems that Mac OS X is not containing such protection:

http://www.macfreek.nl/memory/Kernel_Configuration