message: Update index.html
  Link /doc/filter
diff: @@ -40,7 +40,12 @@ under the License.
  </div>
  <h1>PageSpeed Documentation</h1>
  -
  +<p>
  +PageSpeed improves web page latency by <a href="/doc/filters">changing the resources</a>
  +on that web page to implement
  +<a target="_blank" href="https://developers.google.com/speed/docs/best-practices/rules_intro">
  +web performance best practices</a>
  +</p>
  <div class=columns>
  <div id=guides class=column>
  <h2>Guides</h2>
  