Consider mandating DCO sign-offs

[achrinza]

DCO (Developer Certificate of Origin) is a CLA-alternative that allows a contributor to assert that they have the rights to submit the accompanying code.

DCOs are enforced through a DCO sign-off (via the -s flag in git commit).

The text of DCO can be found here: https://developercertificate.org/

It should be noted that we should explicitly state that they are doing a DCO sign-off by including it as part of our pull request template.