Document the new release process

RELEASING.md

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+# Releasing and publishing the SDK
+
+While the release process can be handled manually, cargo-release has been
+configured to make it more convenient.
+
+By default, [`cargo-release`](https://github.com/crate-ci/cargo-release) assumes
+that no pull request is required to cut a release. However, since the SDK
+repo is set up so that each push requires a pull request, we need to slightly
+deviate from the default workflow. A cargo-xtask has been created to make the
+process as smooth as possible.
+
+The procedure is as follows:
+
+1. Switch to a release branch:
+
+```bash
+git switch -c release-x.y.z
+```
+
+2. Prepare the release. This will update the README.md, prepend the CHANGELOG.md
+   file using `git cliff`, and bump the version in the `Cargo.toml` file.
+
+```bash
+cargo xtask release prepare --execute minor|patch|rc
+```
+
+3. Double-check and edit the changelog and README if necessary. Once you are
+   satisfied, push the branch and open a PR.
+
+```bash
+git push --set-upstream origin/release-x.y.z
+```
+
+4. Pass the review and merge the branch as you would with any other branch.
+
+5. Create tags for your new release, publish the release on crates.io and push
+   the tags:
+
+```bash
+# Switch to main first.
+git switch main
+# Pull in the now-merged release commit(s).
+git pull
+# Create tags, publish the release on crates.io, and push the tags.
+cargo xtask release publish --execute
+```
+For more information on cargo-release: https://github.com/crate-ci/cargo-release