Document that Autoprefixer is required

Closes #14937
[skip sauce]

docs/_includes/getting-started/download.html

@@ -43,4 +43,7 @@
   <h3 id="download-bower">Install with Bower</h3>
   <p>You can also install and manage Bootstrap's Less, CSS, JavaScript, and fonts using <a href="http://bower.io">Bower</a>:</p>
   {% highlight bash %}$ bower install bootstrap{% endhighlight %}
+
+  <h3 id="download-autoprefixer">Autoprefixer required for Less/Sass</h3>
+  <p>Bootstrap uses <a href="https://github.com/postcss/autoprefixer">Autoprefixer</a> to deal with <a href="http://webdesign.about.com/od/css/a/css-vendor-prefixes.htm">CSS vendor prefixes</a>. If you're compiling Bootstrap from its Less/Sass source and not using our Gruntfile, you'll need to integrate Autoprefixer into your build process yourself. If you're using precompiled Bootstrap or using our Gruntfile, you don't need to worry about this because Autoprefixer is already integrated into our Gruntfile.</p>
 </div>