Only require sigs for "release artifacts".

KEYS is not a release artifact.  Neither is README or CHANGES.
Archival happens automatically.

To reduce confusion about how to satisfy the archival requirement, note
that it happens automatically.
Upcase "REQUIRED" for release approval.
Reorder approval criteria to emphasize policy.

Verifying that the release candidate meets ASF policy requirements
cannot be automated and should get first mention.

Additionally, make this passage compatible with releases containing
multiple source packages.
Merge branch 'except_readme_v1'
Merge branch 'auto_archival_v1'
Merge branch 'approval_criteria_v1'
Clarify policy administration.

Rename "Policy Changes" to "Administration".  Require that projects inform the
Board about policy deviations.
Merge branch 'board_admin_v2'