Add a contribution ladder to gittuf

Signed-off-by: Pat Zielinski <[email protected]>

Author: patzielinski

CONTRIBUTOR_LADDER.md

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+# gittuf Contributor ladder
+
+gittuf's contributor ladder is loosely inspired by the OpenSSF [Scorecard
+Contributor Ladder]. This document details how community members may advance to
+more-involved roles within the gittuf project.
+
+- [Roles](#roles)
+    - [Community members](#community-members)
+    - [Maintainers](#maintainers)
+- [Inactive members](#inactive-members)
+
+## Roles
+
+### Community members
+
+Community members engage with gittuf, contributing their time and energy in
+discussions or just generally helping out.
+
+#### Pre-requisites
+
+- Must follow the [OpenSSF Code of Conduct]
+- Must follow the [Contribution Guide]
+
+#### Responsibilities
+
+- Keep it up!
+
+### Maintainers
+
+Maintainers are responsible for the project's overall health. They are the only
+ones who can approve and merge code contributions.
+
+**Defined by:** Listed in the `MAINTAINERS.txt` file in the project repository
+
+#### Pre-requisites
+
+- Reviewed at least 5 substantial PRs to the codebase
+- Reviewed or got at least 10 PRs merged to the codebase
+
+#### Responsibilities
+
+- Demonstrate sound technical judgment
+- Maintain project quality control via code reviews
+- Be responsive to review requests
+- Mentor contributors
+- Participate in gittuf community meetings, if possible
+- Facilitate gittuf community meetings, if possible and comfortable
+
+#### Promotion process
+
+- May self-nominate or be nominated by a maintainer
+    - With no objections from other maintainers
+    - Done through PR to update the `MAINTAINERS.txt` file
+    - In case of nomination, nominating maintainer must comment approval on the PR
+
+## Inactive members
+
+A core principle in maintaining a healthy community is encouraging active
+participation. It is inevitable that a contributor's focus will change over time
+and there is no expectation they'll actively contribute forever.
+
+Any contributor at any level described above may write an issue (or PR, if
+`MAINTAINERS.txt` changes are necessary) asking to step down to a lighter-weight
+tier or to depart the project entirely. Such requests will hopefully come after
+thoughtful conversations with the rest of the team and with sufficient
+forewarning for the others to prepare. However, sometimes "life happens".
+Therefore, the change in responsibilities will be understood to take immediate
+effect, regardless of whether the issue/PR has been acknowledged or merged.
+
+However, should a Maintainer be deemed inactive for a significant period, any
+Community Member or above may write an issue/PR requesting their removal from
+the ranks (and `@mentioning` the inactive contributor in the hopes of drawing
+their attention). The request must receive support (in comments) from a majority
+of Maintainers to proceed.
+
+In all cases, the maintainer's entry in `MAINTAINERS.txt` file will be updated
+to reflect their emeritus status.
+
+[OpenSSF Code of Conduct]: /CODE-OF-CONDUCT.md
+[Contribution Guide]: https://github.com/gittuf/gittuf/blob/main/CONTRIBUTING.md
+[Scorecard Contributor Ladder]: https://github.com/ossf/scorecard/blob/main/CONTRIBUTOR_LADDER.md