Add contribution guide

CONTRIBUTING.md

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+# Contributing
+
+Thank you for considering contributing to `flutter_web_auth_2`! We appreciate your time and effort. To maintain a healthy and collaborative open-source community, please follow the guidelines outlined in this document.
+
+## Table of Contents
+
+- [Code of Conduct](#code-of-conduct)
+- [How to Contribute](#how-to-contribute)
+  - [Setting Up the Development Environment](#setting-up-the-development-environment)
+  - [Making Changes](#making-changes)
+  - [Testing](#testing)
+  - [Documentation](#documentation)
+  - [Pull Requests](#pull-requests)
+- [Community and Communication](#community-and-communication)
+- [License](#license)
+
+## How to Contribute
+
+### Setting Up the Development Environment
+
+1. Fork the repository and clone it to your local machine:
+
+   ```bash
+   git clone https://github.com/ThexXTURBOXx/flutter_web_auth_2.git
+   ```
+
+2. Navigate to the project directory:
+
+   ```bash
+   cd flutter_web_auth_2
+   ```
+
+3. Install project dependencies:
+
+   ```bash
+   melos bootstrap
+   ```
+
+### Making Changes
+
+1. Create a new branch for your feature or bug fix:
+
+   ```bash
+   git checkout -b feature-name
+   ```
+
+2. Make your changes and ensure that your code follows the project's coding style.
+
+### Documentation
+
+1. Update the documentation to reflect your changes.
+
+2. If you're adding a new feature, make sure to include relevant information in the documentation.
+
+### Pull Requests
+
+1. Commit your changes and push the branch to your fork:
+
+   ```bash
+   git add .
+   git commit -m "Your descriptive commit message"
+   git push origin feature-name
+   ```
+
+2. [Open a pull request](https://github.com/ThexXTURBOXx/flutter_web_auth_2/compare) against the `main` branch of the original repository.
+
+3. Clearly describe your changes in the pull request, including any motivation and context.
+
+4. Your pull request will be reviewed by maintainers, and additional changes may be requested.
+
+## License
+
+By contributing to `flutter_web_auth_2`, you agree that your contributions will be licensed under the project's [MIT License](LICENSE).
+
+Happy coding!