Add zwave_js dev docs readme (#47621)

homeassistant/components/zwave_js/README.md

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+# Z-Wave JS Architecture
+
+This document describes the architecture of Z-Wave JS in Home Assistant and how the integration is connected all the way to the Z-Wave USB stick controller.
+
+## Architecture
+
+### Connection diagram
+
+![alt text][connection_diagram]
+
+#### Z-Wave USB stick
+
+Communicates with devices via the Z-Wave radio and stores device pairing.
+
+#### Z-Wave JS
+
+Represents the USB stick serial protocol as devices.
+
+#### Z-Wave JS Server
+
+Forward the state of Z-Wave JS over a WebSocket connection.
+
+#### Z-Wave JS Server Python
+
+Consumes the WebSocket connection and makes the Z-Wave JS state available in Python.
+
+#### Z-Wave JS integration
+
+Represents Z-Wave devices in Home Assistant and allows control.
+
+#### Home Assistant
+
+Best home automation platform in the world.
+
+### Running Z-Wave JS Server
+
+![alt text][running_zwave_js_server]
+
+Z-Wave JS Server can be run as a standalone Node app.
+
+It can also run as part of Z-Wave JS 2 MQTT, which is also a standalone Node app.
+
+Both apps are available as Home Assistant add-ons. There are also Docker containers etc.
+
+[connection_diagram]: docs/z_wave_js_connection.png "Connection Diagram"
+[//]: # (https://docs.google.com/drawings/d/10yrczSRwV4kjQwzDnCLGoAJkePaB0BMVb1sWZeeDO7U/edit?usp=sharing)
+
+[running_zwave_js_server]: docs/running_z_wave_js_server.png "Running Z-Wave JS Server"
+[//]: # (https://docs.google.com/drawings/d/1YhSVNuss3fa1VFTKQLaACxXg7y6qo742n2oYpdLRs7E/edit?usp=sharing)