Add device registry introduction (#81)

docs/device_registry_index.md

@@ -0,0 +1,25 @@
+---
+title: Device Registry
+sidebar_label: Introduction
+---
+
+The device registry is a registry where Home Assistant keeps track of devices that exposes entities. Any entity that is added to Home Assistant through a config entry and follows entity registry requirements can be put in the registry.
+
+Assigning an entity to the device registry is done by having a device_info property. For hubs you need to manually create a device entry to the device registry.
+
+Being registered has the advantage that there is a single point of identifying entities belonging to one device.
+
+## Defining a device
+
+Attributes for a device are connections, identifiers, manufacturer, model, name and sw_version. For hubs the config entry is also required.
+
+A device is looked up in the registry based on its' identifiers and connections (sets of tuples with keyword and a unique value). Identifiers needs to be common spanning all related entities.
+
+Good sources for identifiers are
+
+ - Serial number of a device
+ - MAC address of a device
+
+Good sources for connections are
+
+- MAC address of a device

website/sidebars.json

@@ -40,6 +40,9 @@
     ],
     "Entity Registry": [
       "entity_registry_index"
+    ],
+    "Device Registry": [
+      "device_registry_index"
     ]
   },
   "Extending Frontend": {