Add README with installation and usage instructions

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

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+# Personal Nix Configuration
+
+A simple Nix configuration for macOS using nix-darwin and home-manager. Largely generated by Claude Code.
+
+## Quick Install
+
+Install directly from GitHub without cloning:
+
+```bash
+# Darwin system configuration
+sudo darwin-rebuild switch --flake github:Logan-Lin/nix-config
+
+# Home Manager configuration  
+home-manager switch --flake github:Logan-Lin/nix-config#yanlin
+```
+
+## What's Included
+
+- **nix-darwin**: System-level macOS configuration
+- **home-manager**: User environment and dotfiles
+- **Neovim**: Configured with treesitter, file explorer, and gruvbox theme
+- **Tmux**: Vim-like keybindings with gruvbox colors
+- **Packages**: LaTeX, Python, btop, fonts, and development tools
+
+## Usage
+
+```bash
+# Update and switch configuration
+nix flake update
+sudo darwin-rebuild switch --flake .
+home-manager switch --flake .#yanlin
+```
+
+## Machines
+
+- `iMac` - iMac configuration
+- `mba` - MacBook Air configuration
+
+For now, both machines use the same base configuration with potential for machine-specific customizations.