docs: add CONTRIBUTING.md guide

[dashitongzhi]

Summary\n\nAdded a CONTRIBUTING.md to help new contributors get started with EverOS:\n\n- Development setup instructions\n- Project structure overview\n- Code style guidelines\n- Testing instructions\n- PR process documentation\n\nThis improves the contributor experience for new community members.

[Copilot]

Pull request overview

Adds a new top-level CONTRIBUTING.md intended to onboard new contributors to EverOS. The document, as committed, is a short generic template covering a few high-level workflow steps and a brief license note rather than the detailed guide described in the PR summary.

Changes:

• Introduces CONTRIBUTING.md with "Getting Started" and "Making Changes" sections.
• Adds a brief License section stating contributions are under the project license.

Comments suppressed due to low confidence (1)

CONTRIBUTING.md:18

• The "Getting Started" and "Making Changes" sections substantially overlap (both end with "Submit a PR" and both include making changes and testing). Consider consolidating into a single workflow section, or differentiating them clearly (e.g., one for initial environment setup, one for the per-change workflow) to avoid redundancy and reader confusion.

## Getting Started

1. Clone the repository
2. Install dependencies
3. Make your changes
4. Run tests
5. Submit a PR

## Making Changes

1. Create a feature branch
2. Make your changes
3. Test thoroughly
4. Submit a PR with description

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