Add README.md

Signed-off-by: Stefan Agner <stefan@agner.ch>

README.md

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+# Home Assistant Yellow
+
+This repository contains the PCB design files of the Home Assistant Yellow.
+The design has been made using the open source design software KiCad 6.0.
+The project got successfully crowd funded on [Crowd
+Supply](https://www.crowdsupply.com/nabu-casa/home-assistant-yellow) in
+October 2021. Home Assistant Yellow continous to be available. For updated
+information on availability check
+[yellow.home-assistant.io](https://yellow.home-assistant.io/).
+
+## Overview
+
+Home Assistant Yellow is a custom PCB and requires a Raspberry Pi Compute
+Module 4 (CM4) to be able to run. The PCB is design to run Home Assistant
+(using Home Assistant OS).
+
+The main features are:
+
+- Raspberry Pi Compute Module 4 (CM4) board-to-board connector
+- Silicon Labs MGM210P 802.15.4 capable radio module
+- M.2 slot with PCIe x1 connected to the CM4 (e.g. for NVMe storage)
+- 12V DC power input
+- Gigabit Ethernet with assembly option for PoE+
+- USB 2.0 Hub
+- TI Audio DAC
+- CP2102N bulit-in USB-to-UART converter for serial console
+
+More information about Home Assistant Yellow can be found at
+[yellow.home-assistant.io](https://yellow.home-assistant.io/).
+
+### System Design Overview
+
+![Home Assistant Yellow System Design Overview](SystemDesign.png)
+