add arwn documentation (#1276)

source/_components/sensor.arwn.markdown

@@ -0,0 +1,30 @@
+---
+layout: page
+title: "ARWN Sensor"
+description: "Instructions how to integrate ARWN within Home Assistant."
+date: 2016-10-20 15:15
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Sensor
+ha_release: 0.31
+---
+
+The `arwn` sensor platform is a client for the
+[Ambient Radio Weather Network](http://github.com/sdague/arwn)
+project. This collects weather station data and makes it available in
+an MQTT subtree.
+
+To use your ARWN setup, you must already have configured the
+[MQTT](mqtt) platform. Then add the following to your
+`configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  platform: arwn
+```
+
+Currently all temperature, barometer, and wind sensors will be
+displayed. Support for rain guage sensors will happen in the future.