Documentation/SubmittingPatches: Reference external DCO

Documentation/SubmittingPatches: Reference external DCO

The new developer-certificate-of-origin text is copied directly from
developercertificate.org [1] into a stand-alone file to avoid
confusion with the separately-licensed supporting text in
SubmittingPatches (following the example of stand-alone license files
like COPYING).  SubmittingPatches now references that file when
explaining the significance of Signed-off-by tags.

My changes to SubmittingPatches are licensed under both the Creative
Commons CC0 1.0 Universal license (see the `license` branch for full
text) and the GPLv2-linux (see the `copying` branch for full text), as
are my previous contributions to SubmittingPatches.  Pick whichever
one works best for you.

[1]: http://developercertificate.org/

Signed-off-by: W. Trevor King <wking@tremily.us>