add official site note

index.html

@@ -69,6 +69,10 @@
     </p>
     <hr>
     <p>
+    <strong class="important">Note:</strong> This site is the official version of 2048. Play it on your phone from <a href="http://git.io/2048">http://git.io/2048.</a> All other versions or sites are derivatives or fakes, and should be used with due caution.
+    </p>
+    <hr>
+    <p>
     Created by <a href="http://gabrielecirulli.com" target="_blank">Gabriele Cirulli.</a> Based on <a href="https://itunes.apple.com/us/app/1024!/id823499224" target="_blank">1024 by Veewo Studio</a> and conceptually similar to <a href="http://asherv.com/threes/" target="_blank">Threes by Asher Vollmer.</a>
     </p>
   </div>