Document release process

RELEASE.md

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+# openlifu-python Release Process
+
+This document describes how to release the `openlifu` Python package. This
+repository is the upstream library for SlicerOpenLIFU and openlifu-app.
+
+It comes down to just making a git tag and publishing a GitHub release. A PyPI
+publishing workflow is automatically triggered.
+
+## Versioning
+
+Use git tags of the form:
+
+```text
+vX.Y.Z
+```
+
+The package version is derived from git metadata by `hatch-vcs`; do not maintain
+a separate hard-coded package version.
+
+## Release steps
+
+1. Check whether the sample database needs a matching release tag. If the
+   database format, bundled example data, protocols, transducers, or
+   compatibility expectations changed, publish or identify the corresponding
+   `openlifu-sample-database` tag before releasing, and update the
+   openlifu-python README to reference the new sample database tag.
+2. Confirm `main` is ready for release and CI is passing.
+3. Create and push the openlifu-python git tag:
+
+   ```bash
+   git checkout main
+   git pull
+   git tag vX.Y.Z
+   git push origin vX.Y.Z
+   ```
+
+4. Draft a GitHub release from the tag.
+5. Publish the GitHub release.
+
+The CD workflow should trigger and the package should automatically become
+available on PyPI.
+
+## Release Branches
+
+This repository does not normally need release branches. Prefer the tag-only
+flow above.
+
+Create a release branch only when an older library line needs maintenance after
+`main` has moved on:
+
+```text
+release/0.21
+  v0.21.1
+  v0.21.2
+```
+
+In that case, merge fixes to `main` first when practical, then cherry-pick them
+to the maintenance branch.
+
+## Downstream Updates
+
+Downstream repositories can update their pins when ready:
+
+- SlicerOpenLIFU pins `openlifu` in
+  `OpenLIFULib/OpenLIFULib/Resources/python-requirements.txt`.
+- OpenLIFU-app gets the `openlifu` version indirectly through its pinned
+  SlicerOpenLIFU `GIT_TAG`.
+
+If this release changes database compatibility, call that out in the GitHub
+release notes.