[plt-scheme] Simple sockets examples

> And since the site is now responding again, here's the actual URL
> to that article:
> 
> http://www.free-soft.org/FSM/english/issue01/sk.html

With Shriram's permission, I have added this tutorial to the Scheme 
Cookbook, at:

http://schemecookbook.org/Cookbook/PingPongTutorial