Add DCO

[caniszczyk]

https://developercertificate.org/

[mattklein123]

Will call this resolved once bot is active, we remove Lyft CLA bot, etc.

[caniszczyk]

@mattklein123 DCO bot is now active (I forgot about the built in one with GitHub now):

https://github.com/organizations/envoyproxy/settings/installations/53244

All PRs now will need DCO (including in flight PRs)

If we do a quick test today, we can remove the CLA today.

[mattklein123]

OK I will figure out how to do this and then add instructions in the contributing guide.

[caniszczyk]

This is done. CLA check is remove, DCO check is enforced.

[mattklein123]

I'm still figuring out how to correctly configure git client. Then will update contributing guide and call this fixed.

[caniszczyk]

@mattklein123

I updated the CONTRIBUTING.md with info: #1673

[caniszczyk]

@mattklein123 if you want it to be automatic, you can add something like this to your .gitconfig

[alias]
amend = commit -s --amend
commit = commit -s

There may be easier ways to do this with git now but I have to check things out.

[mattklein123]

Yeah I'm poking around. We can merge this and I will update again when I figure it out.