Added documentation for Skybell (#3511)

* Added documentation for Skybell

* Changed release version from 0.55 to 0.56

* All the blank lines

* Stray tab

source/_components/sensor.skybell.markdown

@@ -0,0 +1,31 @@
+---
+layout: page
+title: "Skybell Sensor"
+description: "Instructions on how to integrate your Skybell HD devices within Home Assistant."
+date: 2017-10-03 16:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: skybell.png
+ha_category: Sensor
+ha_release: 0.56
+ha_iot_class: "Cloud Polling"
+---
+
+To get your [Skybell.com](https://skybell.com/) sensors working within Home Assistant, please follow the instructions for the general [Skybell component](/components/skybell).
+
+Once you have enabled the [Skybell component](/components/skybell), add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: skybell
+    monitored_conditions:
+      - chime_level
+```
+
+Configuration variables:
+
+- **monitored_conditions** array (*Required*): Conditions to display in the frontend. The following conditions can be monitored.
+  - **chime_level**: Return a value between 0-3, indicating no chime, low, medium, and high respectively.