add require autoload.php to example (fixes #842)

Author: Dominic Bordelon

README.md

@@ -30,6 +30,8 @@ Upgrading from v4.x? Facebook PHP SDK v5.x introduced breaking changes. Please [
 Simple GET example of a user's profile.
 
 ```php
+require_once 'vendor/autoload.php'; // change path as needed
+
 $fb = new \Facebook\Facebook([
   'app_id' => '{app-id}',
   'app_secret' => '{app-secret}',