Repeat warning comment about keeping "What's included" sections in sync

Refs #15238
[skip sauce]

docs/_includes/getting-started/whats-included.html

@@ -30,7 +30,8 @@ bootstrap/
     ├── glyphicons-halflings-regular.ttf
     └── glyphicons-halflings-regular.woff
 {% endhighlight %}
-
+<!-- NOTE: This info is intentionally duplicated in the README.
+Copy any changes made here over to the README too. -->
   <p>This is the most basic form of Bootstrap: precompiled files for quick drop-in usage in nearly any web project. We provide compiled CSS and JS (<code>bootstrap.*</code>), as well as compiled and minified CSS and JS (<code>bootstrap.min.*</code>). CSS <a href="https://developers.google.com/chrome-developer-tools/docs/css-preprocessors">source maps</a> (<code>bootstrap.*.map</code>) are available for use with certain browsers' developer tools. Fonts from Glyphicons are included, as is the optional Bootstrap theme.</p>
 
   <h2 id="whats-included-source">Bootstrap source code</h2>