Add trend sensor docs. (#904)

source/_components/binary_sensor.trend.markdown

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+---
+layout: page
+title: "Trend Binary Sensor"
+description: "Instructions how to integrate Trend binary sensors into Home Assistant."
+date: 2016-09-05 10:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Binary Sensor
+logo: home-assistant.png
+---
+
+The `trend` platform allows you to create sensors which show the trend of numeric `state` or`state_attributes` from other entities.
+
+This sensor requires two updates of the underlying sensor to establish a trend - so it can take some time to show an accurate state.
+
+It can be useful as part of automations, where you want to base an action on a trend.
+
+To enable Trend binary sensors in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+binary_sensor:
+  - platform: trend
+    sensors:
+      solar_angle:
+        friendly_name: 'Sun rising'
+        entity_id: sun.sun
+        sensor_class: light
+        attribute: elevation
+```
+
+Configuration variables:
+
+- **sensors** array (*Required*): List of your sensors.
+  - **friendly_name** (*Optional*): Name to use in the Frontend.
+  - **sensor_class** (*Optional*): The [type/class](/components/binary_sensor/) of the sensor to set the icon in the frontend.
+  - **entity_id** (*Required*): The entity that this sensor tracks.
+  - **attribute** (*Optional*): The attribute of the entity that this sensor tracks. If no attribute is specified - the sensor will track the state.
+  - **invert** (*Optional*): Invert the result (so `true` means decending rather than ascending)
+
+## {% linkable_title Examples %}
+
+In this section you find some real life examples of how to use this sensor.
+
+### {% linkable_title Temperature trend %}
+
+This example indicates true if the temperture is rising:
+
+```yaml
+binary_sensor:
+  - platform: trend
+    sensors:
+      temperature_up:
+        friendly_name: 'Temp increasing'
+        entity_id: sensor.skylight_temperature
+        sensor_class: heat
+```
+
+And this one indicates true if the temperture is falling:
+
+```yaml
+binary_sensor:
+  - platform: trend
+    sensors:
+      temperature_down:
+        friendly_name: 'Temp decreasing'
+        entity_id: sensor.skylight_temperature
+        sensor_class: cold
+        invert: Yes
+```