Add mirror site for South Africa (#953)

src/Packagist/WebBundle/Resources/views/Mirrors/index.html.twig

@@ -12,6 +12,7 @@
 <p>Packagist currently provides mirrors in Europe, North America (Montreal), and Asia (Singapore).</p>
 <p>On top of this, we are aware of the following list of third-party-run mirrors, please refer to their website to see how to use them:</p>
 <ul>
+  <li>Africa, South Africa <a href="https://packagist.co.za/">packagist.co.za</a></li>
   <li>Asia, China <a href="https://pkg.phpcomposer.com/">pkg.phpcomposer.com</a></li>
   <li>Asia, Indonesia <a href="https://packagist.phpindonesia.id/">packagist.phpindonesia.id</a></li>
   <li>Asia, Japan <a href="https://packagist.jp/">packagist.jp</a></li>