Add File sensor docs (#2625)

source/_components/sensor.file.markdown

@@ -0,0 +1,60 @@
+---
+layout: page
+title: "File Sensor"
+description: "Instructions how to integrate sensors which read from files into Home Assistant."
+date: 2017-05-13 12:10
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: file.png
+ha_category: Sensor
+ha_iot_class: "Local Polling"
+ha_release: 0.45
+---
+
+
+The `file` sensor platform reading the entries from a plain-text file and shows the found value. Only the last line of the file is used. This is similar to do `$ tail -n 1 sensor.txt` on the command-line.
+
+To enable the `file` sensor, add the following lines to your `configuration.yaml`:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: file
+    file_path: /home/user/.homeassistant/sensor-data.txt
+```
+
+Configuration variables:
+
+- **file_path** (*Required*): path to file that stores the sensor data.
+- **name** (*Optional*): Name of the sensor to use in the frontend. Defaults to `File`.
+- **unit_of_measurement** (*Optional*): Defines the units of measurement of the sensor, if any.
+- **value_template** (*Optional*): Defines a [template](/topics/templating/) to extract a value from the payload.
+
+## {% linkable_title Examples %}
+
+In this section you find some real life examples of how to use this sensor.
+
+### {% linkable_title Entries as JSON %}
+
+Assuming that the log file contains multiple values formatted as JSON like shown below:
+
+```text
+[...]
+{"temperature": 21, "humidity": 39}
+{"temperature": 22, "humidity": 36}
+```
+
+This would require the following entry in the `configuration.yaml` file to extract the temperature:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: file
+    name: Temperature
+    file_path: /home/user/.homeassistant/sensor.json
+    value_template: {% raw %}'{{ value_json.temperature }}'{% endraw %}
+    unit_of_measurement: '°C'
+```
+