Add link to contributors to footer.

Author: Florian Kissling

docs/_includes/footer.html

@@ -5,7 +5,7 @@
       Select2 is licensed under <a href="https://github.com/select2/select2/blob/master/LICENSE.md">MIT</a>, documentation under <a href="https://creativecommons.org/licenses/by/3.0/">CC BY 3.0</a>.
     </p>
     <p>
-      Maintained by <a href="https://github.com/kevin-brown">Kevin Brown</a> and <a href="https://github.com/ivaynberg">Igor Vaynberg</a>.
+      Maintained by <a href="https://github.com/kevin-brown">Kevin Brown</a> and <a href="https://github.com/ivaynberg">Igor Vaynberg</a> with the help of <a href="https://github.com/select2/select2/graphs/contributors">our contributors</a>.
     </p>
     <ul class="s2-docs-footer-links">
       <li>Currently v4.0.0</li>