release: publish checksums and the signing certificate

Print the artifact SHA-256 and the APK signer certificate in the release
log, attach SHA256SUMS and the public release certificate to the GitHub
release, list the APK and certificate SHA-256 in the release notes, print
the next build checksums, and commit the next build's public certificate
with verification steps in signing/README.md.

signing/README.md

@@ -0,0 +1,40 @@
+# Verifying a subtracks download
+
+subtracks ships through three channels, each with its own signer:
+
+- **GitHub releases and Google Play** — signed with the release key. Every GitHub
+  release attaches `subtracks.apk`, a `SHA256SUMS` file and the public certificate
+  `release-certificate.pem`.
+- **`next` builds** (the `subtracks-next` artifact from CI) — signed with the
+  public debug key committed here as [`next-certificate.pem`](next-certificate.pem).
+- **F-Droid** — re-signed by F-Droid with its own key; see F-Droid's published
+  signing key.
+
+## `next` builds
+
+Certificate SHA-256:
+
+```
+DF:31:3F:18:58:E9:AD:F4:7F:44:65:B2:80:8F:DC:E5:B9:F8:8A:48:35:DC:D0:79:95:19:61:C3:05:05:08:37
+```
+
+To check a `next` APK:
+
+```sh
+apksigner verify --print-certs subtracks-next.apk
+```
+
+The `SHA-256 digest` line must equal the fingerprint above (the certificate is
+committed at [`next-certificate.pem`](next-certificate.pem)).
+
+## GitHub releases
+
+Each release attaches the APK, the checksums and the certificate:
+
+```sh
+sha256sum -c SHA256SUMS
+apksigner verify --print-certs subtracks.apk              # compare with the .pem
+keytool -printcert -file release-certificate.pem          # prints its SHA-256
+```
+
+The signing certificate SHA-256 is also listed in the release notes.