Establish review and merge policy

[awoods]

This issue is to draft a policy providing community guidance on how to move extensions pull-requests forward. Reviewing and merging of extensions pull-requests should be driven by community interest and demand.

[awoods]

Given the current content of the extension's README.md file, it likely makes the most sense to include this policy statement there as well. The following text, or an edited version, can be submitted as a pull-request once #13 has been resolved/merged.

Proposed policy text:

OCFL Community Extensions are driven by community demand and contribution. As such, the transition from an extension pull-request to a registered extension should likewise be driven by the community. This policy defines the process by which extension pull-requests are committed into the repository.

1. A pull-request is submitted per the guidelines described in the "[Organization of this repository](https://github.com/OCFL/extensions#organization-of-this-repository)" section of this document
1. Authors of (legitimate) pull-requests will be added by an owner of the OCFL GitHub organization to the [extension-authors](https://github.com/orgs/OCFL/teams/extension-authors) team
   - The purpose of being added to this team is to enable adding `labels` to their pull-request(s)
1. If a pull-request is submitted in order to facilitate discussion, the `draft` label should be applied by the author
1. If a pull-request is ready for review, it should have a title that is suitable for merge (i.e. not have a title indicating "draft"), and optionally have the `in-review` label applied by the author
1. A pull-request must be merged by an OCFL Editor if the following criteria are met:
   1. At least three community members from three different institutions have "[Approved](https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/approving-a-pull-request-with-required-reviews)" the pull-request
   1. At least two OCFL Editors have "Approved" the pull-request
   1. There are no outstanding "Request changes" on the pull-request
   1. Seven days have passed since the last commit on the pull-request, or all OCFL Editors have approved the pull-request

[awoods]

Updated text for last section:

1. A pull-request must be merged by an OCFL Editor if the following criteria are met:
   1. At least two OCFL Editors have "Approved" the pull-request
   1. At least one other community member has "[Approved](https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/approving-a-pull-request-with-required-reviews)" the pull-request
   1. The approvers represent three distinct organizations
...leave last two points as-is