title: Laravel: how add a custom package to my vendor folder?
body: <p>My laravel <code>vendor</code> folder contains many packages from Packagist.
  Now I need a PHP SDK package of my dutch bank <strong>Rabobank (Omnikassa 2)</strong>.</p>
  
  <p>My bank sends me a zip with a the PHP SDK. But how can I add this zip to my Laravel application if I cannot find this SDK on the Packagist website? So I cannot do <code>composer require</code>.
  The SDK contains a composer.json file.</p>