Add Copyright range

Author: Joe Ray Gregory

index.html

@@ -227,7 +227,7 @@ <h3>Community</h3>
       <a href="https://github.com/purescript/purescript"><i class="fa fa-github-square"></i></a>
       <a href="https://twitter.com/purescript"><i class="fa fa-twitter-square"></i></a>
     </p>
-    <p class="text-center text-muted"><small>&copy; PureScript 2019</small></p>
+    <p class="text-center text-muted"><small>&copy; PureScript 2017–2019</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>