scripts/build-release.sh builds reproducible linux/amd64 and linux/arm64
archives with SHA256SUMS; the release workflow runs it on a v* tag after
vet, tests and govulncheck, and publishes the release. The README now
covers what the tool is, what it does step by step, why each decision is
made, how to deploy and run a mail host, the security model, and
troubleshooting. -h exits 0.
