docs: add CONTRIBUTING.md guide

CONTRIBUTING.md

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+# Contributing to Auto-Fill-Tool
+
+First off, thank you for considering contributing to Auto-Fill-Tool! 
+We welcome contributions of all sizes, from fixing typos to adding new features.
+
+## 🚀 Getting Started
+
+### 1. Fork and Clone
+
+1. Fork the repository on GitHub by clicking the **Fork** button.
+2. Clone your fork locally:
+```bash
+   git clone https://github.com/YOUR-USERNAME/Auto-Fill-Tool.git
+   cd Auto-Fill-Tool
+```
+3. Add upstream remote:
+```bash
+   git remote add upstream https://github.com/sunilkumar2170/Auto-Fill-Tool.git
+```
+
+### 2. Install Dependencies
+```bash
+npm install
+```
+
+### 3. Create a Branch
+```bash
+git checkout -b my-new-feature
+```
+Use descriptive names (e.g., `fix/header-typo`, `feature/dark-mode`, `docs/update-readme`).
+
+## 💻 Development Workflow
+
+```bash
+npm run dev
+```
+Open http://localhost:3000 in your browser.
+
+### Linting
+```bash
+npm run lint
+```
+
+### Testing
+```bash
+npm test
+```
+
+## 📁 Project Structure
+
+- `src/app/` — Next.js pages
+- `src/app/components/` — Reusable React components
+- `src/app/lib/` — Utility functions, Redux slices
+- `public/` — Static assets
+
+## 📬 Submitting a Pull Request
+
+1. Commit your changes:
+```bash
+   git add .
+   git commit -m "feat: add dark mode toggle"
+```
+2. Sync with upstream:
+```bash
+   git fetch upstream
+   git rebase upstream/main
+```
+3. Push to your fork:
+```bash
+   git push origin my-new-feature
+```
+4. Open a PR on: https://github.com/sunilkumar2170/Auto-Fill-Tool
+
+5. **Describe your PR clearly:**
+   - What did you fix or add?
+   - Attach screenshots (before & after)
+   - Mention the issue number (e.g., Closes #17)
+
+## 🤔 Need Help?
+
+Open an issue and ask — we are happy to help!
+
+Happy coding! 🎉