Adding link to citation

_includes/lesson_footer.html

@@ -18,6 +18,8 @@
 	/
 	<a href="{{ site.github.repository_url }}/blob/gh-pages/CONTRIBUTING.md">Contributing</a>
 	/
+	<a href="{{ site.github.repository_url }}/blob/gh-pages/CITATION">Cite</a>
+	/
 	<a href="mailto:{{ site.contact }}">Contact</a>
       </h4>
     </div>