Adds file size sensor (#4589)

* Adds file size sensor

* Ammend Configuration variables

Have amended [as per the docs](https://home-assistant.io/developers/documentation/create_page/#configuration), doesn't look correct in my markdown preview however.

* Update sensor.filesize.markdown

* ⬆️ Bumped ha_release

* Update sensor.filesize.markdown

* Update sensor.filesize.markdown

source/_components/sensor.filesize.markdown

@@ -0,0 +1,31 @@
+---
+layout: page
+title: "File size sensor"
+description: "Component for monitoring the size of a file."
+date: 2018-02-06 14:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: file.png
+ha_category: Sensor
+ha_iot_class: "Local Polling"
+ha_release: 0.64
+---
+
+Component for displaying the size of a file. Note that paths must be added to [whitelist_external_dirs](https://home-assistant.io/docs/configuration/basic/).
+Add to your config:
+
+```yaml
+sensor:
+  - platform: filesize
+    file_paths:
+      - /config/home-assistant_v2.db
+  ```
+
+{% configuration %}
+file_paths:
+  description: The absolute path to the file.
+  required: true
+  type: list of strings
+{% endconfiguration %}