docs: create CONTRIBUTING.md with contribution guidelines

CONTRIBUTING.md

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+# 🤝 Contributing to DevImpact
+
+Thank you for your interest in contributing to DevImpact! We welcome contributions from the community and are excited to work with you.
+
+---
+
+## 📋 Table of Contents
+
+- [Code of Conduct](#code-of-conduct)
+- [Getting Started](#getting-started)
+- [Development Setup](#development-setup)
+- [How to Contribute](#how-to-contribute)
+- [Pull Request Guidelines](#pull-request-guidelines)
+- [Coding Standards](#coding-standards)
+- [Reporting Bugs](#reporting-bugs)
+- [Suggesting Features](#suggesting-features)
+
+---
+
+## 📜 Code of Conduct
+
+By participating in this project, you agree to maintain a respectful and inclusive environment. Please be kind and constructive in all interactions.
+
+---
+
+## 🚀 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 18+
+- pnpm
+
+### Install dependencies
+
+```bash
+pnpm install
+```
+
+### Set up environment variables
+
+Create a `.env` file in the project root:
+
+```
+GITHUB_TOKEN=your_github_personal_access_token
+```
+
+> **Note:** Your GitHub token needs `read:user` and `public_repo` scopes.
+
+### Run the development server
+
+```bash
+pnpm run dev
+```
+
+The app will be available at `http://localhost:3000`.
+
+---
+
+## 🔧 How to Contribute
+
+### 1. Find an issue
+
+- Browse [open issues](https://github.com/O2sa/DevImpact/issues)
+- Look for issues labeled `good first issue` or `help wanted`
+- Comment on the issue to let others know you're working on it
+
+### 2. Create a branch
+
+Always branch from `main`:
+
+```bash
+git checkout main
+git pull upstream main
+git checkout -b feat/your-feature-name
+# or
+git checkout -b fix/your-bug-fix
+```
+
+### 3. Make your changes
+
+- Write clean, readable code
+- Follow the existing code style
+- Add comments where the logic is non-obvious
+
+### 4. Test your changes
+
+Make sure the app runs correctly with your changes:
+
+```bash
+pnpm run dev
+```
+
+### 5. Commit your changes
+
+Use clear, descriptive commit messages:
+
+```bash
+git add .
+git commit -m "feat: add dark mode toggle"
+# or
+git commit -m "fix: correct score calculation for forked repos"
+```
+
+### 6. Push and open a PR
+
+```bash
+git push origin feat/your-feature-name
+```
+
+Then open a Pull Request on GitHub.
+
+---
+
+## ✅ Pull Request Guidelines
+
+- **One PR per issue** — keep changes focused
+- **Reference the issue** in your PR description: `Closes #123`
+- **Write a clear description** of what changed and why
+- **Keep diffs small** — large PRs are harder to review
+- Be responsive to reviewer feedback
+
+---
+
+## 🎨 Coding Standards
+
+- **TypeScript** — use proper types, avoid `any`
+- **Tailwind CSS** — use utility classes; avoid inline styles
+- **Components** — keep them small and focused (single responsibility)
+- **File naming** — use kebab-case for files, PascalCase for components
+
+---
+
+## 🐛 Reporting Bugs
+
+Found a bug? Please open an issue with:
+
+1. A clear title and description
+2. Steps to reproduce the bug
+3. Expected vs actual behavior
+4. Screenshots or error messages if available
+5. Your environment (OS, Node.js version, browser)
+
+---
+
+## 💡 Suggesting Features
+
+Have an idea? Open a feature request issue with:
+
+1. A clear description of the feature
+2. Why it would be useful
+3. Any implementation ideas you have
+
+---
+
+## 🙌 Thank You
+
+Every contribution — no matter how small — makes DevImpact better for everyone. We appreciate your time and effort!