[enh] github ribbon added

searx/templates/index.html

 {% extends "base.html" %}
 {% block content %}
+<a href="https://github.com/asciimoo/searx" class="github">
+    <img style="position: absolute; top: 0; right: 0; border: 0;" src="/static/img/github_ribbon.png" alt="Fork me on GitHub"  class="github"/>
+</a>
 <div class="center">
     <h1>searx</h1>
     <form method="post" action="">