README.md: add initial README

README.md

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+# CloudStack Extensions
+
+This repository contains extensions from the community and third-party
+repositories (TPRs).
+
+> [!NOTE]
+> This is an experimental and WIP repository. The config or manifest json/txt
+> format is to be defined.
+
+## Documentation
+
+https://docs.cloudstack.apache.org/en/latest/adminguide/extensions.html
+
+## Contributing
+
+Mailing lists:
+- [Development Mailing List](mailto:dev-subscribe@cloudstack.apache.org)
+- [Users Mailing List](mailto:users-subscribe@cloudstack.apache.org)
+
+Checkout https://cloudstack.apache.org/developers.html for contributing.
+New extensions can be contributed as pull request to this repository, such that
+they are in unique (lowercase) directory which can contain a README.txt specific
+to the extension.
+
+## License
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License. You may obtain a copy of the
+License at http://www.apache.org/licenses/LICENSE-2.0