Add GitHub link

Author: jgthms

index.html

@@ -41,7 +41,8 @@ <h2 class="header-title">
             A free visual guide to CSS
           </h2>
           <p class="header-subtitle">
-            <strong>Learn by example</strong>: <a href="{{site.url}}">cssreference.io</a> is a free visual guide to CSS. It features the most popular properties, and explains them with illustrated and animated examples.
+            <strong>Learn by example</strong>: <a href="{{site.url}}">cssreference.io</a> is a free visual guide to CSS. It features the most popular properties, and explains them with illustrated and animated examples.<br><br>
+            <a href="https://github.com/jgthms/css-reference">Contribute on GitHub</a>
           </p>
         </div>
       </header>