README: Add common landing page

Author: adityasharad

README.md

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+<h1 align="center">GitHub Universe workshops: Finding security vulnerabilities with CodeQL</h1>
+
+> CodeQL is GitHub's expressive language and engine for code analysis, which allows you to explore source code to find bugs and security vulnerabilities. During these beginner-friendly workshops, you will learn to write queries in CodeQL to find specific security vulnerabilities in open-source code.
+
+## Workshops
+
+This repositories contains multiple workshops, delivered at GitHub Universe in different years. Please choose the workshop you are interested in.
+
+- [GitHub Universe 2021](workshop-2021) - Finding security vulnerabilities in Java with CodeQL
+- [GitHub Universe 2020](workshop-2020) - Finding security vulnerabilities in C/C++ with CodeQL
+
+## :books: Resources
+- Learn more about [CodeQL](https://codeql.github.com)
+
+## License
+
+The code in this repository is licensed under the [MIT License](LICENSE) by GitHub.