docs: add instructions for gitlab pypi release

README.md

@@ -63,3 +63,59 @@ You can either use the cli in various ways:
 
 ## Support
 If you need help submit an issue in the [gitlab repository](https://gitlab.dkrz.de/ch1187/rechunk-data/-/issues).
+
+## Steps to Release a New Package to GitLab
+
+### 1. Make sure all issues are resolved and committed
+
+Nothing to do here if you're already done — just ensure your branch is up-to-date.
+
+### 2. Bump the version number
+
+1. Update your version number (e.g. in `src/rechunk_data/__init__.py`):
+  ```python
+  __version__ = "2504.0.0"  # Format: vYYMM.major.minor
+  ```
+2. Update the `CHANGELOG` with the feautures/fixes etc.
+3. Then commit the change:
+  ```shell
+  git add src/rechunk_data/__init__.py
+  git commit -m "Bump version to 2504.0.0"
+  ```
+
+### 3. Create and annotated tag:
+
+```shell
+git tag -a v2404.1.0 -m "Release v2504.0.0: Adds chunking options and size checks"
+```
+
+### 4. Push the tag and latest commits to GitLab
+```shell
+git push origin main  # or your branch
+git push origin v2504.0.0
+```
+
+### 5. Build the package
+```shell
+python -m pip install build
+python -m build
+```
+It will create a `./dist/` folder with a `.whl` and `.tar.gz`.
+
+### 6. Upload to GitLab's PyPI registry
+
+Install Twine if needed:
+```shell
+python -m pip install twine
+```
+
+Then upload (replace <USERNAME> and <TOKEN> with your GitLab credentials):
+```shell
+twine upload --repository-url https://gitlab.dkrz.de/api/v4/projects/<project_id>/packages/pypi dist/* \
+  -u <USERNAME> -p <GITLAB_PERSONAL_ACCESS_TOKEN>
+```
+
+To find your <project_id>, check your GitLab repo URL like:
+```bash
+https://gitlab.dkrz.de/api/v4/projects/<project_id>
+```
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