Add MQTT tag scanner documentation (#14961)

source/_docs/mqtt/discovery.markdown

@@ -19,6 +19,7 @@ Supported by MQTT discovery:
 - [Locks](/integrations/lock.mqtt/)
 - [Sensors](/integrations/sensor.mqtt/)
 - [Switches](/integrations/switch.mqtt/)
+- [Tag Scanners](/integrations/tag_scanner.mqtt/)
 - [Vacuums](/integrations/vacuum.mqtt/)
 
 To enable MQTT discovery, add the following to your `configuration.yaml` file:

source/_integrations/tag.mqtt.markdown

@@ -0,0 +1,81 @@
+---
+title: "MQTT Tag Scanner"
+description: "Instructions on how to integrate MQTT scanner within Home Assistant."
+ha_category:
+  - Tag Scanner
+ha_release: 0.116
+ha_iot_class: Configurable
+ha_domain: mqtt
+---
+
+The `mqtt` tag scanner platform uses an MQTT message payload to generate tag scanned events.
+
+## Configuration
+
+MQTT scanners are only supported through [MQTT discovery](/docs/mqtt/discovery/), manual setup through `configuration.yaml` is not supported.
+The discovery topic needs to be: `<discovery_prefix>/tag/[<node_id>/]<object_id>/config`.
+
+{% configuration %}
+topic:
+  description: The MQTT topic subscribed to receive tag scanned events.
+  required: true
+  type: string
+value_template:
+  description: "Defines a [template](/docs/configuration/templating/#processing-incoming-data) that returns a tag ID."
+  required: false
+  type: string
+device:
+  description: "Information about the device this device trigger is a part of to tie it into the [device registry](https://developers.home-assistant.io/docs/en/device_registry_index.html)."
+  required: true
+  type: map
+  keys:
+    connections:
+      description: "A list of connections of the device to the outside world as a list of tuples `[connection_type, connection_identifier]`. For example the MAC address of a network interface: `'connections': ['mac', '02:5b:26:a8:dc:12']`."
+      required: false
+      type: [list, map]
+    identifiers:
+      description: A list of IDs that uniquely identify the device. For example a serial number.
+      required: false
+      type: [list, string]
+    manufacturer:
+      description: The manufacturer of the device.
+      required: false
+      type: string
+    model:
+      description: The model of the device.
+      required: false
+      type: string
+    name:
+      description: The name of the device.
+      required: false
+      type: string
+    sw_version:
+      description: The firmware version of the device.
+      required: false
+      type: string
+    via_device:
+      description: 'Identifier of a device that routes messages between this device and Home Assistant. Examples of such devices are hubs, or parent devices of a sub-device. This is used to show device topology in Home Assistant.'
+      required: false
+      type: string
+{% endconfiguration %}
+
+## Examples
+
+In this section, you will find some real-life examples of how to use this sensor.
+
+### Full configuration with tag ID extracted from JSON data
+
+This is an example of a configuration where the tag ID is extracted from a JSON formatted MQTT message.
+To test, you can use the command line tool `mosquitto_pub` shipped with `mosquitto` or the `mosquitto-clients` package to send MQTT messages.
+
+Discover the tag scanner:
+
+```bash
+mosquitto_pub -h 127.0.0.1 -t home-assistant/tag/0AFFD2/config -m '{"topic": "tasmota_0AFFD2/tag_scanned", "value_template": "{{ value_json.PN532.UID }}"}'
+```
+
+Generate tag scanned event:
+
+```bash
+mosquitto_pub -h 127.0.0.1 -t tasmota_0AFFD2/tag_scanned -m '{"Time":"2020-09-28T17:02:10","PN532":{"UID":"E9F35959", "DATA":"ILOVETASMOTA"}}'
+```