docs: create CONTRIBUTING.md with project setup and guidelines (closes #29)

CONTRIBUTING.md

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+# Contributing to DevImpact
+
+Thank you for your interest in contributing to DevImpact! This guide will help you get started.
+
+## Table of Contents
+
+- [Getting Started](#getting-started)
+- [Development Setup](#development-setup)
+- [Project Structure](#project-structure)
+- [Making Changes](#making-changes)
+- [Pull Request Guidelines](#pull-request-guidelines)
+- [Issue Guidelines](#issue-guidelines)
+- [Coding Standards](#coding-standards)
+- [Need Help?](#need-help)
+
+## Getting Started
+
+1. **Fork** the repository on GitHub
+2. **Clone** your fork locally:
+   ```bash
+   git clone https://github.com/your-username/DevImpact.git
+   cd DevImpact
+   ```
+3. **Add the upstream remote**:
+   ```bash
+   git remote add upstream https://github.com/O2sa/DevImpact.git
+   ```
+
+## Development Setup
+
+### Prerequisites
+
+- [Node.js](https://nodejs.org/) (v18 or higher)
+- [pnpm](https://pnpm.io/) package manager
+- A [GitHub Personal Access Token](https://github.com/settings/tokens) with `read:user` and `repo` scopes
+
+### Installation
+
+1. Install dependencies:
+   ```bash
+   pnpm install
+   ```
+
+2. Create a `.env` file in the project root (see `.env.example`):
+   ```
+   GITHUB_TOKEN=your_github_token_here
+   ```
+
+3. Start the development server:
+   ```bash
+   pnpm dev
+   ```
+
+4. Open [http://localhost:3000](http://localhost:3000) in your browser.
+
+## Project Structure
+
+```
+DevImpact/
+├── app/              # Next.js App Router pages and API routes
+├── components/       # Reusable React components
+├── lib/              # Utility functions, GitHub API client, scoring logic
+├── types/            # TypeScript type definitions
+├── .github/          # Issue templates, PR template, workflows
+├── tailwind.config.ts
+├── next.config.js
+└── tsconfig.json
+```
+
+### Tech Stack
+
+- **Framework**: Next.js 16+ (App Router)
+- **Language**: TypeScript
+- **Styling**: Tailwind CSS
+- **UI Components**: Radix UI, Lucide React icons
+- **Charts**: Recharts
+- **API**: GitHub GraphQL API via Octokit
+
+## Making Changes
+
+1. **Sync your fork** with the latest upstream changes:
+   ```bash
+   git fetch upstream
+   git checkout main
+   git merge upstream/main
+   ```
+
+2. **Create a feature branch** from `main`:
+   ```bash
+   git checkout -b feat/your-feature-name
+   ```
+
+3. **Make your changes** and test them locally.
+
+4. **Run the linter** before committing:
+   ```bash
+   pnpm lint
+   ```
+
+5. **Commit your changes** with a clear message:
+   ```bash
+   git commit -m "feat: add your feature description"
+   ```
+
+6. **Push** to your fork and open a Pull Request.
+
+### Commit Message Format
+
+Use descriptive commit messages with a prefix:
+
+- `feat:` for new features
+- `fix:` for bug fixes
+- `docs:` for documentation changes
+- `refactor:` for code refactoring
+- `style:` for formatting changes (no logic change)
+- `test:` for adding or updating tests
+
+## Pull Request Guidelines
+
+- Reference the related issue using `Fixes #<issue_number>` in the PR description
+- Keep PRs focused on a single change
+- Make sure the linter passes (`pnpm lint`)
+- Test your changes locally before submitting
+- Fill out the PR template provided
+- Be responsive to review feedback
+
+## Issue Guidelines
+
+We use several issue templates:
+
+- **Bug Report** - for reporting bugs
+- **Feature Request** - for suggesting new features
+- **Feature Change Request** - for modifying existing features
+- **Documentation** - for documentation improvements
+- **Refactoring** - for code quality improvements
+
+When opening an issue, please use the appropriate template and provide as much detail as possible.
+
+## Coding Standards
+
+- **TypeScript**: Use proper types. Avoid `any` where possible.
+- **Components**: Keep components small and focused. Use the `components/` directory for reusable UI elements.
+- **Styling**: Use Tailwind CSS utility classes. Follow the existing patterns in the codebase.
+- **API calls**: Use the existing GitHub API client in `lib/` rather than creating new API integrations.
+- **File naming**: Use kebab-case for files (e.g., `compare-form.tsx`).
+
+## Need Help?
+
+- Check the [open issues](https://github.com/O2sa/DevImpact/issues) for tasks you can work on
+- Look for issues labeled `good first issue` for beginner-friendly tasks
+- Open a new issue if you have questions or suggestions
+
+## License
+
+By contributing to DevImpact, you agree that your contributions will be licensed under the [MIT License](LICENSE).