README: document Sigsum proofs

README.md

@@ -157,6 +157,38 @@ go install filippo.io/age/cmd/...@latest
 
 Help from new packagers is very welcome.
 
+### Verifying the release signatures
+
+If you download the pre-built binaries, you can check their
+[Sigsum](https://www.sigsum.org) proofs, which are like signatures with extra
+transparency: you can cryptographically verify that every proof is logged in a
+public append-only log, so you can hold the age project accountable for every
+binary release we ever produced. This is similar to what the [Go Checksum
+Database](https://go.dev/blog/module-mirror-launch) provides.
+
+```
+cat << EOF > age-sigsum-key.pub
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIEjDYFJ4WVbxRLcgbppmPaMFS/Wbq+1r9cl4qdJTyRVL
+EOF
+cat << EOF > sigsum-trust-policy.txt
+log 154f49976b59ff09a123675f58cb3e346e0455753c3c3b15d465dcb4f6512b0b https://poc.sigsum.org/jellyfish
+witness poc.sigsum.org/nisse 1c25f8a44c635457e2e391d1efbca7d4c2951a0aef06225a881e46b98962ac6c
+witness rgdd.se/poc-witness  28c92a5a3a054d317c86fc2eeb6a7ab2054d6217100d0be67ded5b74323c5806
+group  demo-quorum-rule all poc.sigsum.org/nisse rgdd.se/poc-witness
+quorum demo-quorum-rule
+EOF
+
+curl -JO "https://dl.filippo.io/age/v1.2.0?for=darwin/arm64"
+curl -JO "https://dl.filippo.io/age/v1.2.0?for=darwin/arm64&proof"
+
+go install sigsum.org/sigsum-go/cmd/[email protected]
+sigsum-verify -k age-sigsum-key.pub -p sigsum-trust-policy.txt \
+    age-v1.2.0-darwin-arm64.tar.gz.proof < age-v1.2.0-darwin-arm64.tar.gz
+```
+
+You can learn more about what's happening above in the [Sigsum
+docs](https://www.sigsum.org/getting-started/).
+
 ## Usage
 
 For the full documentation, read [the age(1) man page](https://filippo.io/age/age.1).