WAQI sensor (#1451)

* WAQI sensor description.

* Rework as Markdown table.

source/_components/sensor.waqi.markdown

@@ -0,0 +1,41 @@
+---
+layout: page
+title: "World Air Quality Index"
+description: "Instructions how to setup World Air Quality Index sensor in Home Assistant."
+date: 2016-11-17 06:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: smtp.png
+ha_category: Sensor
+ha_release: 0.32
+ha_iot_class: "Local Polling"
+---
+
+The `waqi` sensor platform will query [World Air Quality Index](http://aqicn.org) service to check AQI value for a specific set of locations. The resulting indexes will be added to the Home Assistant as sensor outputs.
+
+To enable this sensor, add the following lines to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: waqi
+    locations:
+      - beijin
+```
+
+Configuration variables:
+
+- **locations** (*Required*): a list of location names to look for air quality data. In case a specific location has multiple registered stations all of them will be added to Home Assistant
+
+The value reported is an overall AQ index for the location. The values of the index can be interpreted as following:
+
+AQI | Status | Description
+------- | :----------------: | ----------
+0 - 50  | **Good** | Air quality is considered satisfactory, and air pollution poses little or no risk
+51 - 100  | **Moderate** | Air quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution
+101 - 150 | **Unhealthy for Sensitive Groups** | Members of sensitive groups may experience health effects. The general public is not likely to be affected
+151 - 200 | **Unhealthy** | Everyone may begin to experience health effects; members of sensitive groups may experience more serious health effects
+201 - 300 | **Very unhealthy** | Health warnings of emergency conditions. The entire population is more likely to be affected
+301+ | **Hazardous** | Health alert: everyone may experience more serious health effects