Merge branch 'dco' into contributing

* dco:
  developer-certificate-of-origin: Add v1.1 from the Linux Foundation

Signed-off-by: W. Trevor King <wking@tremily.us>
CONTRIBUTING: Update now that the DCO is a separate file

Also add little teasers explaining what is in each file.

My changes to CONTRIBUTING are licensed under both the Creative
Commons CC0 1.0 Universal license (see the `license` branch for full
text).

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