Add documentation for new platform luftdaten sensor (#3857)

* Add documentation for luftdaten sensor

* Minor adjustments

source/_components/sensor.luftdaten.markdown

@@ -0,0 +1,48 @@
+---
+layout: page
+title: "Luftdaten Sensor"
+description: "Instructions how to setup Luftdaten sensor in Home Assistant."
+date: 2017-11-01 10:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: home-assistant.png
+ha_category: Health
+ha_release: 0.57
+ha_iot_class: "Cloud Polling"
+---
+
+The `luftdaten` sensor platform will query the open data API of [luftdaten.info](http://luftdaten.info) to monitor air quality and other weather data from a specific (self build) sensor station.
+
+- To get the ID of a particle sensor you need to select it on the [Feinstaub map](http://deutschland.maps.luftdaten.info/) and find it in the sidebar (Column "Sensor ID").
+- To get the ID of a temperature/humidity sensor you need to find it on the map hosted on [Madavi](https://www.madavi.de/sensor/feinstaub-map-dht/).
+
+To enable this sensor, add the following lines to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: luftdaten
+    sensorid: 3123
+    monitored_conditions:
+      - P1
+      - P2
+  - platform: luftdaten
+    sensorid: 155
+    monitored_conditions:
+      - temperature
+      - humidity
+```
+
+Configuration variables:
+
+- **sensorid** (*Required*): The ID of the sensor.
+- **monitored_conditions** (*Required*): A list of conditions you want to monitor. The following conditions can be monitored:
+  - `P1` for particle sensors (particles 10 microns and below).
+  - `P2` for particle sensors (particles 2.5 microns and below).
+  - `temperature` for weather sensors.
+  - `humidity` for weather sensors.
+- **name** (*Optional*): Name of the sensor to use in the frontend. Defaults to `Luftdaten Sensor`.
+- **resource** (*Optional*): The URL of the API endpoint. Usually this has not to be changed. The default is `https://api.luftdaten.info/v1/sensor/`.
+- **verify_ssl** (*Optional*): Verify SSL connection. Defaults to `true`.