document the php-gd extension requirement (#643)

Author: niol

_docs/website/index.html

@@ -94,7 +94,7 @@ <h2>Requirements</h2>
             <div class="documentation-entry">
                 <p>selfoss is not a hosted service. It has to be installed on your own webserver. This webserver must fulfill the following requirements (which are available from most providers)</p>
                 <ul>
-                    <li>PHP 5.3 or higher</li>
+                    <li>PHP 5.3 or higher with the php-gd extension enabled</li>
                     <li>MySQL, PostgreSQL or Sqlite</li>
                     <li>Apache Webserver (ngnix and lighttpd also possible)</li>
                 </ul>