README: update for repository archival

[prestist]

Summary

Update README to indicate this repository has been archived and Butane has been merged into the Ignition repository.

Dependencies

This PR should be merged after the following have landed:

• coreos/ignition#2235 -- Butane merge PR
• FESCo approval of the F45 Change Request
• Ignition 2.27.0 release (first release with native Butane support)

Archival steps

After merging this PR:

1. Update repo description to: [ARCHIVED] Butane has been merged into https://github.com/coreos/ignition
2. Archive the repo via GitHub settings

Tracking

• coreos/fedora-coreos-tracker#2006

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