Readme: add link to GH preview

closes #942

Author: cagix

readme.md

@@ -17,6 +17,15 @@ has_license: true
   [daringfireball.net]: https://daringfireball.net/2020/04/cobol_programming_coding
 
 
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+**Read Online**:
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+[![GitHub Actions Workflow Status](https://img.shields.io/github/actions/workflow/status/Programmiermethoden-CampusMinden/Prog2-Lecture/gfm.yaml?branch=master&style=flat&label=GitHub%20Preview)](https://github.com/Programmiermethoden-CampusMinden/Prog2-Lecture/blob/_gfm/readme.md)
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+---
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 ## Kursbeschreibung
 
 Sie haben letztes Semester in **Prog1** die *wichtigsten* Elemente und Konzepte der