docs: Add release_process.md

docs/release_process.md

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+# Release process
+
+To release new versions of one or more packages in the workspace:
+
+1. Create a local branch and make these changes:
+   1. Update the version of each package you want to release in the
+      corresponding `Cargo.toml`. Follow semver.
+   2. If needed, update the dependency version in packages that depend
+      on the updated package. For example, if you are releasing changes
+      to `uguid` that require a major version bump, the dependency on
+      `uguid` in `gpt_disk_types` should be updated. Ditto for a minor
+      version bump if the dependent package requires any of the new
+      functionality. Patch version bumps do not require a change in the
+      dependent package. Note that in cargo's version of semver, `0.x.y`
+      treats `x` as the major version number.
+   3. Update the changelogs.
+2. Commit those changes. The commit message subject must start with
+   `release:`. Without that prefix, the automatic part of the release
+   process will not run.
+3. Push the branch and create a PR.
+4. When the PR is reviewed and merged, the automatic release process
+   will kick off.
+
+## The automatic part
+
+The rest of the release process is triggered by
+`.github/workflows/release.yml`, which runs when commits are pushed to
+the main branch. It runs `cargo xtask auto_publish`.
+
+The `auto_publish` command gets the local version of each package and,
+if necessary, creates a remote git tag and publishes to crates.io. If
+the git tag already exists, that part is skipped. If the crates.io
+release already exists, that part is skipped.
+
+## Crates.io token
+
+Releasing to crates.io requires an API token. That is passed to the job
+via a repository secret called `CARGO_REGISTRY_TOKEN`. The secret is
+configured in the repo settings. When creating an API token, restrict
+the scope to `publish-update`.