New component: JSON MQTT Device Tracker (#2416)

* New component: JSON MQTT Device Tracker

* Update device_tracker.mqtt_json.markdown

* Update device_tracker.mqtt_json.markdown

* JSON keys have changed in dev PR

* Update device_tracker.mqtt_json.markdown

source/_components/device_tracker.mqtt_json.markdown

@@ -0,0 +1,39 @@
+---
+layout: page
+title: "JSON MQTT Device Tracker"
+description: "Instructions how to use JSON MQTT to track devices in Home Assistant."
+date: 2017-04-12 20:41
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: mqtt.png
+ha_category: Presence Detection
+ha_version: 0.44
+---
+
+
+The `mqtt_json` device tracker platform allows you to detect presence by monitoring an MQTT topic for new locations. To use this platform, you specify a unique topic for each device.
+
+To use this device tracker in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+device_tracker:
+  - platform: mqtt
+    devices:
+      paulus_oneplus: location/paulus
+      annetherese_n4: location/annetherese
+```
+
+Configuration variables:
+
+- **devices** (*Required*): List of devices with their topic.
+- **qos** (*Optional*): The QoS level of the topic.
+
+This platform receives JSON formatted payloads containing GPS information, for example:
+```json
+{"longitude": 1.0,"gps_accuracy": 60,"latitude": 2.0,"battery_level": 99.9}
+```
+Where `longitude` is the longitude, `latitude` is the latitude, `gps_accuracy` is the accuracy in meters, `battery_level` is the current battery level of the device sending the update.
+`longitude` and `latitude` are required keys, `gps_accuracy` and `battery_level` are optional.