Documentation/DeveloperCertificateOfOrigin: Add with LF copyright

Luis R. Rodriguez [1] has been trying to get information about the
licensing of the Linux kernel's DCO [2] for a while now [3,4], and it
looks like the Linux Foundation just made their view explicit [5,6,7].
This clarifies the copyright and licensing of Linus' two patches:

* 857a183 Update DCO ("signoff") rules to 1.1
* 991bd2e Start documenting the sign-off procedure in SubmittingPatches

From the whois information, developercertificate.org is pretty recent:

  $ whois developercertificate.org
  Domain Name: DEVELOPERCERTIFICATE.ORG
  Domain ID: D170689185-LROR
  Creation Date: 2014-01-15T02:54:55Z
  Updated Date: 2014-01-17T22:11:12Z
  ...
  Name Server: NS2.LINUX-FOUNDATION.ORG
  Name Server: NS1.LINUX-FOUNDATION.ORG

The new DeveloperCertificateOfOrigin text is copied directly from
developercertificate.org [7] 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.

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://www.do-not-panic.com/
[2]: https://www.kernel.org/doc/Documentation/SubmittingPatches
[3]: http://thread.gmane.org/gmane.linux.kernel/1397613
[4]: http://thread.gmane.org/gmane.linux.kernel/1492612
[5]: http://article.gmane.org/gmane.linux.kernel.wireless.general/118696
[6]: http://article.gmane.org/gmane.linux.kernel/1635433
[7]: http://developercertificate.org/

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