add official site note

index.html

@@ -69,6 +69,10 @@ <h1 class="title">2048</h1>
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+    <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>
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