Improve quickstart example, include autoloader and install instructions

Author: clue

theme/homepage.html.twig

@@ -41,6 +41,13 @@
                 <div class="welcome__example">
             {% markdown %}
 ```php
+<?php
+
+// $ composer require react/http react/socket # install example using Composer
+// $ php example.php # run example on command line, requires no additional web server
+
+require __DIR__ . '/vendor/autoload.php';
+
 $http = new React\Http\HttpServer(function (Psr\Http\Message\ServerRequestInterface $request) {
     return React\Http\Message\Response::plaintext(
         "Hello World!\n"