require DCO for all new commits

[GeorgLink]

This issue is to activate protobot/dco (or similar bot) to check that all commits have a sign-off. The CHAOSS Project Charter requires that all contributions are signed-off.

For users of the git command line interface, a sign-off is accomplished with the -s as part of the commit command: git commit -s -m 'This is a commit message'

For users of the GitHub interface, a sign-off is accomplished by writing Signed-off-by: Your Name <YourName@example.org> into the commit comment field. This can be automated by using a browser plugin like scottrigby/dco-gh-ui

[GeorgLink]

DCO is required.