Create README.md

Author: simonerom

datasource/README.md

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+# W3bstream Supported Data Availability (DA) Infra
+This folder contains the necessary components and interfaces to support DA infrastructure in W3bstream.
+
+## Contents
+
+- **[DA Interface](./datasource.go):** Defines the interface for Data Availability implementations.
+- **[Example Implementation (Postgres)](./postgres.go):** A sample implementation of the DA interface using Postgres.
+
+## Contributing
+
+We welcome contributions to expand the range of Data Availability implementations supported by W3bstream. If you have developed a new implementation, please follow these steps to contribute:
+
+1. **Fork the Repository**: Create a personal copy of the repository by forking it.
+2. **Create a Branch**: Create a new branch for your implementation.
+3. **Implement and Test**: Develop your implementation and ensure it passes all tests.
+4. **Submit a Pull Request (PR)**: Once your implementation is complete, submit a PR to the main repository.
+
+We will review your PR and merge it upon approval. Thank you for contributing to W3bstream!
+
+## Getting Started
+
+To get started with the existing implementations, you can explore the [`datasource.go`](./datasource.co) and [`postgres.go`](./postgres.go) files for interface definitions and examples.
+
+## Contact
+
+For any questions or support, please reach out to our team on [Discord](https://iotex.io/devdiscord) or open an issue in this repository.
+