docs: add RELEASE.md describing the release-please flow

Document how this SDK is released: conventional commits drive a release-please
release PR; merging it tags and publishes. Covers the repo's registry, secret,
version file, the one-time repo setting, and the retry path, and links to the
gateway compatibility doc for the cross-repo policy.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: njbrake

RELEASE.md

@@ -0,0 +1,40 @@
+# Releasing
+
+This SDK is versioned independently of the otari gateway, with its own semver.
+Releases are automated with [release-please](https://github.com/googleapis/release-please).
+
+## How a release happens
+
+1. Merge changes to `main` using [Conventional Commits](https://www.conventionalcommits.org/)
+   (`feat:`, `fix:`, etc.). This includes the gateway codegen's regeneration PRs
+   and ordinary shell PRs.
+2. release-please opens or updates a single **release PR** that bumps the version
+   in `pyproject.toml` (`[project].version`) and writes `CHANGELOG.md`.
+3. Review and merge the release PR. That tags the release and creates a GitHub
+   Release.
+4. The same workflow run (`.github/workflows/release-please.yml`, gated on
+   `release_created`) builds the package and publishes it to PyPI.
+
+## Configuration
+
+- **Registry:** PyPI (`otari`).
+- **Auth:** PyPI trusted publishing via OIDC (`environment: pypi`, `id-token: write`).
+  No API token secret is stored; the trusted publisher must be configured on PyPI.
+- **Version file:** `pyproject.toml` `[project].version` (the only place to change
+  the version; do not edit it by hand, release-please owns it).
+
+## Prerequisites (one time, repo settings)
+
+- Enable **Settings to Actions: "Allow GitHub Actions to create and approve pull
+  requests"** so release-please can open its release PR.
+- Configure the PyPI trusted publisher for this repo and the `pypi` environment.
+
+## If the publish fails
+
+The release tag and GitHub Release already exist, so re-run the failed `publish`
+job from the Actions tab to retry publishing the same version. Avoid cutting a
+release by hand; the automated path keeps `pyproject.toml`, the tag, and the
+changelog in sync.
+
+See the gateway's [SDK release coordination and compatibility](https://github.com/mozilla-ai/otari/blob/main/docs/sdk-compatibility.md)
+for the cross-repo policy, the spec-version model, and the end-to-end flow.