Add attribution for logo (#128)

Closes #127

Author: paf31

Diff for: index.html

@@ -222,6 +222,8 @@ <h3>Book</h3>
       <a href="https://twitter.com/purescript"><i class="fa fa-twitter-square"></i></a>
     </p>
     <p class="text-center text-muted"><small>&copy; PureScript 2017</small></p>
+    <p class="text-center text-muted"><small>The PureScript logo by Gareth Hughes is used under the terms of the Creative Commons Attribution 4.0 license.
+</small></p>
   </footer>
 
 </body>