Implement a DCO-style contributor policy

Implement a DCO-style contributor policy

This is a way for contributors to attest to their right to contribute
and the license of the contribution.

This Contributor Policy is based on the Developer's Certificate of
Origin 1.1 from
<http://web.archive.org/web/20060524185355/http://www.osdlab.org:80/newsroom/press_releases/2004/2004_05_24_dco.html>,
which is is (C) 2005 Open Source Development Labs, Inc. and was
originally licensed under CC BY-SA 2.5 and also said that "if you
modify you must use a name or title distinguishable from "Developer's
Certificate of Origin" or "DCO" or any confusingly similar name."

Being that this is a slightly modified version I changed the name to
be a generic "Contributor Policy" and also upgraded the license to
the newer CC BY-SA 4.0.

Signed-off-by: Jason Self <j@jxself.org>