Add docs on how to install Bootstrap via composer

Closes #16113
Closes #16114

[skip sauce]

README.md

@@ -28,7 +28,8 @@ Five quick start options are available:
 - Clone the repo: `git clone https://github.com/twbs/bootstrap.git`.
 - Install with [Bower](http://bower.io): `bower install bootstrap`.
 - Install with [npm](https://www.npmjs.com): `npm install bootstrap`.
-- Install with [Meteor](https://www.meteor.com/): `meteor add twbs:bootstrap`.
+- Install with [Meteor](https://www.meteor.com): `meteor add twbs:bootstrap`.
+- Install with [Composer](https://getcomposer.org): `composer require twbs/bootstrap`.
 
 Read the [Getting started page](http://getbootstrap.com/getting-started/) for information on the framework contents, templates and examples, and more.
 

docs/_includes/getting-started/download.html

@@ -54,6 +54,10 @@ <h2 id="download-npm">Install with npm</h2>
     <li><code>style</code> - path to Bootstrap's non-minified CSS that's been precompiled using the default settings (no customization)</li>
   </ul>
 
+  <h2 id="download-composer">Install with Composer</h2>
+  <p>You can also install and manage Bootstrap's Less, CSS, JavaScript, and fonts using <a href="https://getcomposer.org">Composer</a>:</p>
+  {% highlight bash %}$ composer require twbs/bootstrap{% endhighlight %}
+
   <h2 id="download-autoprefixer">Autoprefixer required for Less/Sass</h2>
   <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>