doc(RELEASE.md): write down release process

This is what I've been doing. It may need optimization, but that's
easier once the process is written down.

RELEASE.md

@@ -0,0 +1,49 @@
+# Release process for the Radicle CI broker
+
+Lars follows this process to make a release of the CI broker.
+
+1. Make sure all the changes needed for the release have been made.
+   - check open issues for anything urgent
+   - check open patches for anything that should be included
+   - check Zulip discussions for anything not captured in issues
+1. Make sure the code in `main` can be deployed to `ci0` and CI runs
+   successfully for all repositories.
+   - `cibtool --db ci-broker.db trigger --all`
+1. Start a branch for the release.
+   - `git switch -c release`
+1. Run `make`, ensure it's successful. This builds the code and runs
+   the test suite.
+1. Update version number in `Cargo.toml`, run `make semver` to verify
+   the new version number follows semantic versioning.
+1. Run `cargo update` to make sure `Cargo.lock` has the new crate version
+   number.
+1. Update new version in `debian/changelog` as well.
+   - `dch -v a.b.c-1 New version.`
+   - `dch -r ''`
+1. Update `NEWS.md` basedon what's changed since the previous release.
+   - `git log -p vA.B.C..`
+1. Run `rad ci` to make sure CI runs successfully.
+1. Commit changes.
+   - `git commit -am "chore: prepare radicle-ci-broker release A.B.C."`
+1. Check if publishing would work.
+   - `cargo publish --dry-run`
+1. Check `deb` building works. Throw away results.
+   - `debian/build-deb`
+   - `rm ../*_*`
+1. Push patch, wait for CI to succeed.
+   - `git push rad HEAD:refs/patches`
+1. Merge patch, push merge.
+   - `git switch main`
+   - `git merge release`
+   - `git push rad HEAD`
+1. Create release tag.
+   - `git tag -sam "radicle-ci-broker release version A.B.C"` vA.B.C
+1. Publish crate.
+   - `cargo publish`                                                                                                                                    
+1. Build `deb`, import into the Radicle APT repository.
+   - `debian/build-deb`                                                                                
+1. Announce on Zulip in the `#annonce` channel in the topic for Radicle CI
+   releases. Include `NEWS.md` snipppet for release, changing the top heading
+   to mention "Radicle CI broker".
+1. Upgrade my CI nodes to new releasse.
+1. Update my other Radicle CI components if needed.