Adding warning about multiple open files.

libraries/SD/examples/ReadWrite/ReadWrite.pde

@@ -38,7 +38,8 @@ void setup()
   }
   Serial.println("initialization done.");
   
-  // open a file:
+  // open the file. note that only one file can be open at a time,
+  // so you have to close this one before opening another.
   myFile = SD.open("test.txt", FILE_WRITE);
   
   // if the file opened okay, write to it: