Add composer.json with league/commonmark, guzzlehttp/guzzle, phpmailer/phpmailer; wire autoloader into bootstrap; document de-librairisation strategy and PHP extension setup

app/bootstrap.php

@@ -4,6 +4,9 @@
  * Simple configuration for website
  */
 
+// Composer autoloader
+require_once __DIR__ . '/../vendor/autoload.php';
+
 // Define application root
 define('APP_ROOT', __DIR__);