Clarify documentation license to be CC BY 3.0

Signed-off-by: Chris Aniszczyk <zx@twitter.com>

docs/index.html

@@ -1923,7 +1923,7 @@ Lorem ipsum dolar sit amet illo error <a href="#" title="below">ipsum</a> verita
         <p class="pull-right"><a href="#">Back to top</a></p>
         <p>
           Designed and built with all the love in the world <a href="http://twitter.com/twitter" target="_blank">@twitter</a> by <a href="http://twitter.com/mdo" target="_blank">@mdo</a> and <a href="http://twitter.com/fat" target="_blank">@fat</a>.<br />
-          Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0" target="_blank">Apache License v2.0</a>.
+          Code licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0" target="_blank">Apache License v2.0</a>. Documentation licensed under <a href="http://creativecommons.org/licenses/by/3.0/">CC BY 3.0</a>.
         </p>
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