Add version sensor docs (#3163)

* Add version sensor docs

* Fix typo

source/_components/sensor.version.markdown

@@ -0,0 +1,67 @@
+---
+layout: page
+title: "Version Sensor"
+description: "Instructions how to integrate a version sensor into Home Assistant."
+date: 2017-08-10 10:30
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Sensor
+ha_iot_class: "Local Pushing"
+logo: home-assistant.png
+ha_release: 0.52
+---
+
+
+The `version` sensor platform is displaying the current version of Home Assistant in the frontend.
+
+To enable this sensor, add the following lines to your `configuration.yaml` file for a GET request:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: version
+```
+
+Configuration variables:
+
+- **name** (*Optional*): Name of the sensor. Defaults to `Current Version`.
+
+## {% linkable_title Alternatives %}
+
+This sensor is an alternative to the existing solutions to achieve the same result through various platforms. Remember that you can easily get the installed version on the command line.
+
+```bash
+$ hass --version
+```
+
+Or go to the <img src='/images/screenshots/developer-tool-about-icon.png' alt='service developer tool icon' class="no-shadow" height="38" /> **Info** section of the **Developer Tools**.
+
+A [`command_line`](/components/sensor.command_line/) with [`hass`](/docs/tools/hass/) to display your current version.
+
+```yaml
+sensor:
+  - platform: command_line
+    name: Version
+    command: "/home/homeassistant/bin/hass --version"
+```
+
+It's also possible to ready a file called `.HA_VERSION` which is located in your Home Assistant [configuration](/docs/configuration/) folder.
+
+```yaml
+sensor:
+  - platform: command_line
+    name: Version
+    command: "cat /home/homeassistant/.homeassistant/.HA_VERSION"
+```
+
+You might think that a [`rest` sensor](/components/sensor.rest/) could work to but it will not as Home Assistant is not ready when the sensor get initialized.
+
+```yaml
+sensor:
+  - platform: rest
+    resource: http://IP_ADDRESS:8123/api/config
+    name: Current Version
+    value_template: '{{ value_json.version }}'
+```