docs: Add additional way to signoff commits

[jpayne3506]

Adds an additional way to signoff commits within CONTRIBUTING.md

[jpayne3506]

Closing this loop: #750 (comment)

Also, I leveraged git rebase -i HEAD~4 --signoff to get a signoff on all of my commits as I did not know about the DCO. Perhaps this would be a good addition for https://github.com/project-copacetic/copacetic/blob/main/CONTRIBUTING.md#i-didnt-sign-my-commit-now-what?

yes! I think it would be good to document this

[ashnamehrotra]

LGTM, thank you!