docs: Add CONTRIBUTING.md with contribution guidelines

Added guidelines for contributing to the project, including reporting issues, development setup, code style, commit messages, pull requests, and code of conduct.

Author: hellsontime

CONTRIBUTING.md

@@ -0,0 +1,71 @@
+# Contributing
+
+Thank you for your interest in contributing to scdl.
+
+## Reporting Issues
+
+If you find a bug or have a feature request, please open an issue and include:
+
+- A clear description of the problem
+- Steps to reproduce (if applicable)
+- Expected vs actual behavior
+- Your environment (OS, Go version)
+
+Keep issues focused and reproducible.
+
+## Development Setup
+
+Requirements:
+
+- Go (latest stable version recommended)
+
+Clone the repository:
+
+```bash
+git clone git@github.com:hellsontime/scdl.git
+cd scdl
+go mod tidy
+```
+
+Build:
+
+```bash
+go build ./...
+```
+
+## Code Style
+
+- Follow standard Go formatting (`gofmt`)
+- Use clear, explicit error handling
+- Avoid unnecessary dependencies
+- Keep functions small and focused
+- Prefer simple and readable implementations
+
+Before submitting:
+
+```bash
+make check
+make check-coverage
+```
+
+## Commit Messages
+
+This project follows Conventional Commits.
+
+Examples:
+
+- `feat: add progressive stream detection`
+- `fix: handle expired signed url`
+- `docs: update README usage section`
+- `refactor: simplify transcoding resolver`
+
+## Pull Requests
+
+- Keep PRs small and focused
+- Describe what changed and why
+- Reference related issues if applicable
+- Ensure the project builds successfully
+
+## Code of Conduct
+
+By participating in this project, you agree to abide by the Code of Conduct.