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>

diff --git a/ATTESTATION.md b/ATTESTATION.md
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+Contributor Policy
+==================
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I have
+    the right to submit it under the license indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best of
+    my knowledge, is covered under an appropriate license and I have
+    the right under that license to submit that work with
+    modifications, whether created in whole or in part by me, under
+    the same license (unless I am permitted to submit under a
+    different license), as indicated in the file; or
+
+(c) The contribution was provided directly to me by some other person
+    who certified (a), (b) or (c) and I have not modified it; and
+
+(d) In the case of each of (a), (b), or (c), I understand and agree
+    that this project and the contribution are public and that a
+    record of the contribution (including all personal information I
+    submit with it, including my sign-off) is maintained indefinitely
+    and may be redistributed consistent with this project or the
+    license indicated in the file.
+
+To indicate you understand and agree to this add a line to your
+contribution saying:
+
+    Signed-off-by: Your Name <user@host.root>
+
+using your real name and email address (sorry, no pseudonyms or
+anonymous contributions.)
+
+Git makes it easy to add the Signed-off-by.
+
+First, set up git with your real name and email address using commands
+such as these:
+
+    git config --global user.name "Your Name"
+    git config --global user.email user@host.root
+
+Once that is done just use -s or --signoff when you commit:
+
+    git commit -s or git commit --signoff
+
+Contributions that do not have a Signed-off-by cannot be accepted.
+
+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 licensed
+under the Creative Commons Attribution-ShareAlike 4.0 License:
+<https://creativecommons.org/licenses/by-sa/4.0/>