Docs for uptime sensor (#3614)

* Docs for uptime sensor

* Update platform name

source/_components/sensor.uptime.markdown

@@ -0,0 +1,30 @@
+---
+layout: page
+title: "Uptime Sensor"
+description: "Instructions how to integrate an uptime sensor into Home Assistant."
+date: 2017-10-13 10:30
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Sensor
+ha_iot_class: "Local Pushing"
+logo: home-assistant.png
+ha_release: 0.56
+---
+
+
+The `uptime` sensor platform displays the time since the last Home Assistant restart.
+
+To enable this sensor, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: uptime
+```
+
+Configuration variables:
+
+- **name** (*Optional*): Name of the sensor. Defaults to `Uptime`.
+- **unit_of_measurement** (*Optional*): Units for uptime measurement in either `days` or `hours`.  Defaults to `days`.