Add release flow information

CONTRIBUTING.md

@@ -199,7 +199,47 @@ If you add a new question, please notify us by adding a [matching issue](https:/
 
 ## Release flow
 
-...
+_This section is intended for Polars maintainers._
+
+Polars releases Rust crates to [crates.io](https://crates.io/crates/polars) and Python packages to [PyPI](https://pypi.org/project/polars/).
+
+New releases are marked by an official [GitHub release](https://github.com/pola-rs/polars/releases) and an associated git tag. We utilize [Release Drafter](https://github.com/release-drafter/release-drafter) to automatically draft GitHub releases with release notes.
+
+### Steps
+
+The steps for releasing a new Rust or Python version are similar. The release process is mostly automated through GitHub Actions, but some manual steps are required. Follow the steps below to release a new version.
+
+Start by bumping the version number in the source code:
+
+1. Check the [releases page](https://github.com/pola-rs/polars/releases) on GitHub and find the appropriate draft release. Note the version number associated with this release.
+2. Make sure your fork is up-to-date with the latest version of the main Polars repository, and create a new branch.
+3. Bump the version number.
+
+- _Rust:_ Update the version number in all `Cargo.toml` files in the `polars` directory and subdirectories. You'll probably want to use some search/replace strategy, as there are quite a few crates that need to be updated.
+- _Python:_ Update the version number in [`py-polars/Cargo.toml`](https://github.com/pola-rs/polars/blob/master/py-polars/Cargo.toml#L3) to match the version of the draft release.
+
+4. From the `py-polars` directory, run `make build` to generate a new `Cargo.lock` file.
+5. Create a new commit with all files added. The name of the commit should follow the format `release(<language>): <Language> Polars <version-number>`. For example: `release(python): Python Polars 0.16.1`
+6. Push your branch and open a new pull request to the `master` branch of the main Polars repository.
+7. Wait for the GitHub Actions checks to pass, then squash and merge your pull request.
+
+Directly after merging your pull request, release the new version:
+
+8. Go back to the [releases page](https://github.com/pola-rs/polars/releases) and click _Edit_ on the appropriate draft release.
+9. On the draft release page, click _Publish release_. This will create a new release and a new tag, which will trigger the GitHub Actions release workflow ([Python](https://github.com/pola-rs/polars/actions/workflows/create-python-release.yaml) / [Rust](https://github.com/pola-rs/polars/actions/workflows/release-rust.yaml)).
+10. Wait for all release jobs to finish, then check [crates.io](https://crates.io/crates/polars)/[PyPI](https://pypi.org/project/polars/) to verify that the new Polars release is now available.
+
+### Troubleshooting
+
+It may happen that one or multiple release jobs fail. If so, you should first try to simply re-run the failed jobs from the GitHub Actions UI.
+
+If that doesn't help, you will have to figure out what's wrong and commit a fix. Once your fix has made it to the `master` branch, re-trigger the release workflow by updating the git tag associated with the release. Note the commit hash of your fix, and run the following command:
+
+```shell
+git tag -f <version-number> <commit-hash> && git push -f origin <version-number>
+```
+
+This will update the tag to point to the commit of your fix. The release workflows will re-trigger and hopefully succeed this time!
 
 ## License