[KED-3049] Update contributors guide to include guidance on DCO

[antonymilne]

Description

As per kedro-org/kedro#1167 and kedro-org/kedro#1177, now that we're part of LF, we need all commits to be signed-off. I'm not a fan of this, but after spending far too much time on it I've come up with this workflow as the simplest possible one for now.

1. make sign-off: this creates a git hook that will add the Signed-off-by line to all commit message. This is a better solution than aliasing git commit for various reasons
2. require: members: false for the DCO GitHub app configuration. This means that signed commits (i.e. GPG or S/MIME signed, not signed-off) by members of kedro-org do not need to have the Signed-off-by line. In practice, this is very helpful because it means that any commits maintainers make on GitHub (applying code review suggestions, merging in main, etc.) will get ✅ for DCO check.

Possible future improvements

• Enable allowRemediationCommits to make it easier for users to retroactively sign off commits
• Maybe fork DCO app so that the "Set DCO to pass" button is available to all? (Might defeat the point of the check though...)

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

[rashidakanchwala]

Thanks LGTM!

[studioswong]

awesome work for setting up the docs and the sign off workfow 👍