Change homepage GitHub link label

Summary: Makes it a bit more obvious where this link goes.

Reviewed By: passy

Differential Revision: D25639460

fbshipit-source-id: d2eed6bf2bcb0ac3b6cb9de148e62e65ab61266a

website/src/pages/index.js

@@ -150,7 +150,7 @@ export default function Index() {
               className="learnmore"
               href="https://github.com/facebook/flipper"
               target="_blank">
-              Learn more
+              Explore the source on GitHub
             </a>
           </div>
         </div>