docs(README): add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,58 @@
+# Contributing to Ciyue
+
+Thank you for your interest in contributing to Ciyue!
+
+## Developing
+
+- Make your changes with clear, descriptive commit messages.
+- Ensure your code passes all tests.
+
+### Requirements
+
+- An editor, such as [VS Code](https://code.visualstudio.com/)
+- Flutter development environment. Follow [the official installation guide](https://docs.flutter.dev/get-started/install)
+
+### Using justfile
+
+We use [just](https://github.com/casey/just) as a command runner to simplify common development tasks.
+
+If you don't have just installed, follow the [installation guide](https://github.com/casey/just#installation).
+
+Common commands:
+
+```bash
+$ just build_runner      # Run code generation
+$ just make-migrations   # Generate database migrations
+$ just icon              # Generate app icons
+```
+
+### Build
+
+```bash
+$ flutter build android
+$ flutter build windows
+$ flutter build linux
+```
+
+### Running
+
+```bash
+$ flutter run
+```
+
+## Translations
+
+We welcome contributions to help translate Ciyue into more languages!
+
+[Translate Ciyue on Weblate](https://hosted.weblate.org/engage/ciyue/)
+
+Weblate makes it easy to collaborate on translations online. Please do not edit `.arb` files directly.
+
+## Reporting Issues
+
+- Search existing issues before opening a new one.
+- Provide detailed information to help us reproduce and fix the issue.
+
+## Final
+
+We appreciate your contributions!

README.md

@@ -65,6 +65,10 @@ Thanks to their sponsorship for supporting the development of Ciyue!
 |爱发电用户_1b0db|
 |爱发电用户_nW9j|
 
+## Contributing
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md) for guidelines on how to contribute to this project.
+
 ## Contributors
 
 [![Contributor](https://contrib.rocks/image?repo=mumu-lhl/Ciyue)](https://github.com/mumu-lhl/Ciyue/graphs/contributors)

README_CN.md

@@ -64,6 +64,10 @@
 |爱发电用户_1b0db|
 |爱发电用户_nW9j|
 
+## 贡献指南
+
+请参阅 [CONTRIBUTING.md](./CONTRIBUTING.md) 以了解如何为本项目做出贡献。
+
 ## 贡献者
 
 <a href="https://github.com/mumu-lhl/Ciyue/graphs/contributors">