Add release instructions to repo

RELEASE.md

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+# Releasing Kaleido
+
+Follow the steps in this document to release a new version of Kaleido.
+
+## Overview
+
+- Versions are determined automatically from git tags by `setuptools-git-versioning`. There is no hard-coded version string to update. Pushing a `v*` tag triggers CI, which tests and publishes the package to TestPyPI.
+- Publishing to PyPI and creating a GitHub release are done manually.
+
+## Release steps
+
+1. Create release branch titled `release-vX.Y.Z`
+    - We try to follow [semantic versioning guidelines](https://semver.org) as much as possible
+2. Update the changelog:
+    - Review the items in `CHANGELOG.md` under the "Unreleased" header
+    - Ensure all items follow this format:
+        - `- Description of change [[#XXXX](https://github.com/plotly/Kaleido/pull/XXXX)]`
+        - ...followed by `with thanks to @username for the contribution!` if the PR is from a community contributor
+    - Add any missing items (PRs which were merged since the last release)
+    - PRs which don't change the contents of the built package (e.g. changes to the README, etc.) and minor dependency upgrades don't need to be mentioned
+    - Add a header `## vX.Y.Z` above the unreleased items, under `## Unreleased`
+    - Commit and push the `CHANGELOG.md` updates
+    - _Normally, the changelog is the only file that needs to be updated when making a release. The package version is determined automatically as explained in the overview above._
+3. Open a PR into `main`, wait for tests to pass, get approval, and then merge into `main`.
+4. On your local machine, check out `main`, pull, and tag the release branch merge commit with the version number:
+    ```
+    git checkout main
+    git pull
+    git tag vX.Y.Z
+    git push origin vX.Y.Z
+    ```
+5. Build the release artifacts:
+
+    - [Install `uv`](https://docs.astral.sh/uv/getting-started/installation/) if not already installed on your machine
+
+    - Ensure your git environment inside the `Kaleido/` directory is totally clean, i.e. `git status` returns the following:
+
+        ```
+        On branch main
+        Your branch is up to date with 'origin/main'.
+
+        nothing to commit, working tree clean
+        ```
+
+    - If needed, stash or delete/move files to get to a clean environment. This is important because `setuptools-git-versioning` will not generate the correct version number if you have untracked or changed files present.
+
+    - Build the package:
+        ```
+        cd src/py/
+        uv build
+        ```
+    - Two artifacts will be added to `src/py/dist`. Ensure that they have the correct filenames:
+        - `kaleido-X.Y.Z-py3-none-any.whl`
+        - `kaleido-X.Y.Z.tar.gz`
+6. Release on GitHub:
+    - Go to https://github.com/plotly/Kaleido/releases and click "Draft a new release"
+    - Select the `vX.Y.Z` tag you created previously
+    - Copy and paste the relevant changelog section into the release notes
+    - Upload the `.whl` and `.tar.gz` build artifacts
+7. Release on PyPI:
+    - Install the `twine` Python package if not already installed (`pip install twine`)
+    - `cd` into the `dist/` directory
+    - Use `twine` to upload the artifacts to PyPI:
+        ```
+        twine upload kaleido-X.Y.Z*
+        ```
+    - You will need to enter an API token proving you have permission to publish
+
+That's it! Make sure to visit https://pypi.org/project/kaleido to verify that the new package version is available.