DCO check has been enabled org-wide

[kevin wang]

Hello all contributors,

According to CNCF's IP policy (https://github.com/cncf/foundation/blob/master/charter.md#11-ip-policy), all projects shall use either CLA (Contributor License Agreements) or DCO (Developer Certificate of Origin).

For easier use, the KubeEdge maintainers have enabled DCO check org-wide. 

To sign DCO in your PR, simply use `git commit -s -m "your commit message"` when commiting your changes.

Please note that with DCO check enabled, all pull requests without DCO signing will be blocked from merging.

Refs: 

CNCF IP policy: https://github.com/cncf/foundation/blob/master/charter.md#11-ip-policy

DCO: https://github.com/apps/dco

Relevant issue: https://github.com/kubeedge/kubeedge/issues/2509

Thanks,

Kevin Wang on behalf of KubeEdge maintainers.