docs: add developer.md

developer.md

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+# Release a new version of the software
+
+## PyPi
+
+For a step-by-step video guide check out the [release video](https://www.youtube.com/watch?v=tsAWDumpcW8)
+
+1. Check that the publishing action creating the wheels and source distribution appear correctly.
+2. Go to Actions tab and workflow "Bump version".
+3. Run the workflow manually by clicking the "Run workflow" button and adjusting the parameters.
+    - Set version bump level (i.e. "patch")
+    - Set new version "X.Y.Z" w/o "v" prefix
+    - Disable dry run, after you are confident that the workflow is working correctly (i.e. running dry run once).
+4. Optionally, check the publishing action again for the new version.
+5. Manually, run the publishing action by clicking the "Run workflow" button in order to publish to Test-PyPi.
+6. Optionally, inspect the URL returned after Test-PyPi uploading to check the package.
+   Check that sdist and wheel are available for the new version.
+7. Go to the Releases page.
+   Select "Draft a new release".
+8. Pick the tag created by the "Bump version" workflow (here with a "v" prefix) and click "Generate release notes".
+   Tick "Set as the latest release" if this is the latest release and "Create a discussion" in the Category "Announcements".
+9. Click "Publish release".
+10. A build and publish to (Test)-PyPi workflow will be triggered automatically it should again upload sdist and wheel to PyPi.
+
+## Conda
+
+1. Wait a few hours for automatic PR https://github.com/conda-forge/pylhe-feedstock following the PyPi release.
+2. Review that there are no dependency changes and merge the PR.