index: Link "material" -> "contributing"

This raises the visibility on how you can get the source code, by restoring
the link which was removed in:

  commit ad60f32b7d8eac7868789de2b85bf402dca0c574
  Author: Greg Wilson <gvwilson@third-bit.com>
  Date:   Sat Dec 15 06:51:39 2012 -0500

    Made the home page a little more interesting...