update client software link

(adcportal.com has been terminated)

README

@@ -4,5 +4,5 @@ For the official documentation, bugs and other information, please visit:
 http://www.uhub.org/
 
 For a list of compatible ADC clients, see:
-http://www.adcportal.com/wiki/index.php/ADC_Software_List#Client_Software
+http://en.wikipedia.org/wiki/Advanced_Direct_Connect#Client_software