This is preferable to avoid license proliferation and to make it easy to
find documentation of license compatibility.  This change has been
approved by Aaron Sauers <asauers@anl.gov>.

Contributing guidelines now include the Developer Certificate of Origin,
though our workflow does not require the explicit Signed-off-by tags
adopted by some open source projects (mostly those with mailing list
patch workflows).

  http://developercertificate.org/