Add sensor.postnl documentation (#5231)

* Add sensor.postnl documentation

* Update ha_release

source/_components/sensor.postnl.markdown

@@ -0,0 +1,47 @@
+---
+layout: page
+title: PostNL Sensor
+description: "Instructions on how to set up PostNL sensors within Home Assistant."
+date: 2017-04-22 08:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: postnl.png
+ha_category: Sensor
+ha_release: 0.69
+ha_iot_class: "Cloud Polling"
+---
+
+The `postnl` platform allows one to track deliveries by [PostNL](https://www.postnl.nl) (Dutch Postal Services). To use this sensor, you need a [PostNL Account](https://jouw.postnl.nl). It is possible to add multiple accounts to your Home Assistant configuration.
+
+## {% linkable_title Configuration %}
+
+To enable this sensor, add the following lines to your `configuration.yaml`:
+
+```yaml
+sensors:
+  - platform: postnl
+    username: POSTNL_USERNAME
+    password: POSTNL_PASSWORD
+```
+
+{% configuration %}
+name:
+  description: Sensor name
+  required: false
+  default: "postnl"
+  type: string
+username:
+  description: Account username of jouw.postnl.nl
+  required: true
+  type: string
+password:
+  description: Account password of jouw.postnl.nl
+  required: true
+  type: string
+{% endconfiguration %}
+
+<p class='note warning'>
+This component is not affiliated with PostNL and retrieves date from the endpoints of the mobile application. Use at your own risk.
+</p>