Update Samba's DCO license in compliance with CC-BY-SA 4.0

Update Samba's DCO license in compliance with CC-BY-SA 4.0

The text of "Samba's Developer Certificate of Origin" is copyrighted
and licensed CC-BY-SA.  Add notice for compliance with CC-BY-SA 4.0.

Signed-off-by: Bradley M. Kuhn <bkuhn@sfconservancy.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Simo Sorce <idra@samba.org>
Reviewed-by: Ralph Böhme <slow@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Oct 20 18:10:50 UTC 2020 on sn-devel-184