Add nest binary sensor docs

source/_components/binary_sensor.nest.markdown

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+---
+layout: component
+title: "Nest thermostat sensor"
+description: "Instructions how to integrate Nest thermostats sensors within Home Assistant."
+date: 2016-01-26 08:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: nest_thermostat.png
+ha_category: Binary Sensor
+---
+
+
+The `nest` binary sensor platform let you monitor various states of a thermostat from [Nest](https://nest.com).
+
+To set it up, add the following information to your `configuration.yaml` file:
+
+```yaml
+sensor:
+  platform: nest
+  monitored_conditions:
+    - 'fan'
+    - 'hvac_ac_state',
+    - 'hvac_aux_heater_state',
+    - 'hvac_heat_x2_state',
+    - 'hvac_heat_x3_state',
+    - 'hvac_alt_heat_state',
+    - 'hvac_alt_heat_x2_state',
+    - 'hvac_emer_heat_state',
+    - 'online'
+```
+Configuration variables:
+
+- **monitored_conditions** array (*Required*): States to monitos.
+
+You must have the [Nest Thermostat](https://home-assistant.io/components/thermostat.nest/) entity configured to use this sensor.
+