Add threshold sensor docs (#1511)

source/_components/binary_sensor.threshold.markdown

@@ -0,0 +1,42 @@
+---
+layout: page
+title: "Threshold Binary Sensor"
+description: "Instructions how to integrate threshold binary sensors into Home Assistant."
+date: 2016-11-26 12:10
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: home-assistant.png
+ha_category: Binary Sensor
+ha_iot_class: "Local Polling"
+ha_release: 0.34
+---
+
+
+The `threshold` binary sensor platform is consuming the state from another sensor. If the value is below (`lower`) or higher (`upper`) than the given threshold then state of this sensor change..
+
+It's an alternative to the template binary sensor's `value_template:` to get the abnormal/too high/too low states.
+
+```yaml
+{% raw %}{{ states.sensor.furnace.state > 2.5 }}{% endraw %}
+```
+
+To enable the threshold sensor, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: threshold
+    threshold: 15
+    type: lower
+    entity_id: sensor.random
+```
+
+Configuration variables:
+
+- **entity_id** (*Required*): The entity to monitor. Only [sensors](/components/sensor/) are supported.
+- **threshold** (*Required*): The value which is the threshold.
+- **type** (*Required*): `lower` if the value needs to be below the threshold or `upper` if higher.
+- **name** (*Optional*): Name of the sensor to use in the frontend. Defaults to `Stats`.
+