Added docs for DWD WarnApp Sensor (#3062)

* Added docs for dwdwarnapp

* Updated documentation

As suggested by Landrash

source/_components/sensor.dwdwarnapp.markdown

@@ -0,0 +1,43 @@
+---
+layout: page
+title: "DWD Warn Weather"
+description: "Instructions on how to integrate Deutsche Wetter Dienst weather warnings into Home Assistant."
+date: 2017-07-26 22:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+#logo: dwdwarnapp.png
+ha_category: Weather
+ha_release: 0.51
+ha_iot_class: "Cloud Polling"
+---
+
+The `dwdwarnapp` sensor platform uses the [Deutsche Wetter Dienst (DWD)](http://www.dwd.de) as a source for current and advance warnings.
+
+- A name is optional but if multiple regions are used a name will be required.
+- The sensor checks for new data every 15 minutes.
+
+To add the DWD WarnApp sensor to your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: dwdwarnapp
+    region_name: Hansestadt Hamburg
+```
+
+To get the region name:
+- Find your region here: `https://www.dwd.de/DE/wetter/warnungen_landkreise/warnWetter_node.html?ort=Hamburg`
+- Verify if you find any warning for your region here: `https://www.dwd.de/DWD/warnungen/warnapp_landkreise/json/warnings.json?jsonp=loadWarnings`
+
+The warning level is between 0 (no danger) and 4 (warnings of extreme weather):
+- Warnungen vor extremem Unwetter (Stufe 4)
+- Unwetterwarnungen (Stufe 3)
+- Warnungen vor markantem Wetter (Stufe 2)
+- Wetterwarnungen (Stufe 1)
+
+Configuration variables:
+
+- **region_name** (*Optional*): The region name string as identified from the DWD website.  If not given, defaults to Hansestadt Hamburg.
+- **name** (*Optional*): The name you would like to give to the warnapp sensor.