chore(docs): add release process documentation (#25)

relates to STACKITSDK-176

Author: rubenhoenle

RELEASE.md

@@ -0,0 +1,39 @@
+# Release
+
+## Release cycle
+
+A release of the whole SDK should be created at least every 2 weeks. 
+
+## Release creation
+
+### Single service
+
+The SDK is split into all the different STACKIT [services](https://github.com/stackitcloud/stackit-sdk-java/tree/main/services), each of them having their own version numbers.
+
+**Checklist before releasing a single service**
+
+- [ ] Changelog entries were added to the `CHANGELOG.md` in the root directory of the repository (see [here](https://github.com/stackitcloud/stackit-sdk-java/blob/43cca15713638623ce56f58a4b93836c548cb05c/CHANGELOG.md))
+- [ ] Changelog entries were added to the `CHANGELOG.md` file of the service to be released (see e.g. [here](https://github.com/stackitcloud/stackit-sdk-java/blob/43cca15713638623ce56f58a4b93836c548cb05c/services/resourcemanager/CHANGELOG.md))
+
+**Releasing a single service**
+
+1. Update the VERSION file of the corresponding service
+2. When the PR is merged to main the tag is created automatically
+
+### Whole SDK
+
+**Checklist before releasing the whole SDK**
+
+- [ ] Date was set/updated in the `CHANGELOG.md` file in the root directory of the repository (see [here](https://github.com/stackitcloud/stackit-sdk-java/blob/43cca15713638623ce56f58a4b93836c548cb05c/CHANGELOG.md))
+
+**Releasing the whole SDK**
+
+> [!IMPORTANT]
+> Consider informing / syncing with the team before creating a new release.
+
+1. Check out latest main branch on your machine
+2. Create git tag: `git tag release-YYYY-MM-DD`
+3. Push the git tag: `git push origin --tags`
+4. Copy the changelog entries for the new release from the `CHANGELOG.md` file in the root directory of the repository (see [here](https://github.com/stackitcloud/stackit-sdk-java/blob/43cca15713638623ce56f58a4b93836c548cb05c/CHANGELOG.md)) to your clipboard.
+5. Go to the [releases page](https://github.com/stackitcloud/stackit-sdk-java/releases) on GitHub and create a new release. Select the git tag you just created.
+6. Before creating the GitHub release, add the *Highlights* heading at the top of the markdown description and paste the changelog entries from your clipboard (see [previous releases](https://github.com/stackitcloud/stackit-sdk-java/releases/tag/release-2025-09-09) to see what it should look like). Then create and publish the GitHub release.