add README for GitHub Actions .github/workflows directory

.github/workflows/README.md

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+This directory stores workflow scripts that are picked up by GitHub Actions to automatically perform test installations of V-pipe on Mac OS and Linux systems and run end-to-end tests by executing tutorials with real example data.
+This ensures successful installation and reproducible execution on different systems.
+For each update of V-pipe these workflow scripts are automatically executed and report about installation problems or troubles on the test data