change README to archive note

[davidhewitt]

Change Summary

Changes the README on the main branch to be an archive note.

Once this PR is merged, I will also disable pull-requests to main.

We have the release-2.41 branch where any future backports can be pushed. Otherwise, all development from this point forwards will proceed in the pydantic repository.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1850 will not alter performance

_{Comparing dh/archive-note (4bcf9f7) with main (396499c)}

Summary

✅ 163 untouched

[Viicos]

If we want to ship a future patch release from this repo, we should wait before merging or this updated readme would get included in the wheel archives.

[davidhewitt]

I think it won't be a problem. The idea is that we will build the future patch release from a tag on the release-2.41 branch which will not include this change.