docs: add README with Raspberry Pi deployment guide

## Description
Expand the README with a step-by-step guide for deploying NanoClaw on a Raspberry Pi.

## Requirements
- [ ] Hardware requirements (RPi 4, 2GB+ RAM)
- [ ] OS setup (Raspberry Pi OS Lite)
- [ ] Installation steps (pip install or git clone)
- [ ] Configuration guide
- [ ] Systemd service file for auto-start

## Skills
Markdown, Raspberry Pi, Linux