Adding link to source

_includes/footer.html

@@ -12,6 +12,8 @@
     <h4>
       Copyright &copy; 2016 Software Carpentry Foundation
       /
+      <a href="{{ site.repo }}">Source</a>
+      /
       <a href="{{ site.root }}/license/">License</a>
       /
       <a href="mailto:{{site.email}}">Contact</a>