Docs: Links training doc on performant watch faces to blog post on same topic.

<h2>You should also read</h2>

<ul>

  <li><a href="{@docRoot}design/wear/watchfaces.html">Watch Faces for Android Wear</a></li>

  <li><a href="http://android-developers.blogspot.com/2014/12/making-performant-watch-face.html">

Making a performant watch face</a></li>

</ul>

</div>

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