MAINTAINERS_GUIDE: introduce guidance on maintainer expectations

[vbatts]

This is adopted from https://github.com/opencontainers/runc/blob/602c85fdc6c73d614213fc1759f8e710e54047ca/MAINTAINERS_GUIDE.md

Notably:

• making the "runc" into a more generic "this project"
• dropping the concept of "chief maintainer"
• formatting to a sentence per line

Signed-off-by: Vincent Batts [email protected]

As this effectively is putting into effect expectations for current maintainers, I would expect this PR to get review from maintainers and only merge with a 2/3 majority quorum , rather than the simple 2 LGTM cc @opencontainers/image-spec-maintainers:

• [ ] Jon Johnson (@jonjohnsonjr)
• [ ] Jonathan Boulle (@jonboulle)
• [ ] Stephen Day (@stevvooe)
• [ ] Vincent Batts (@vbatts)
• [ ] Aleksa Sarai (@cyphar)

[vbatts]

good comments. Updated.