add official site note
add official site note

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