Updated all serial in setup examples with a note about the serial check

libraries/Ethernet/examples/TwitterClient/TwitterClient.ino

@@ -54,7 +54,9 @@ void setup() {
 
  // Open serial communications and wait for port to open:
   Serial.begin(9600);
-  while(!Serial) ;
+  // this check is only needed on the Leonardo:
+  while (!Serial) ;
+ ;
 
   // attempt a DHCP connection:
   Serial.println("Attempting to get an IP address using DHCP:");