Add docs for Prowl. (#3194)

* Add docs for Prowl.

* Update 'ha_release'

source/_components/notify.prowl.markdown

@@ -0,0 +1,43 @@
+---
+layout: page
+title: "Prowl"
+description: "Instructions how to add Prowl notifications to Home Assistant."
+date: 2017-08-17 18:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: prowl.png
+ha_category: Notifications
+ha_release: 0.52
+---
+
+
+The `prowl` platform uses [Prowl](https://www.prowlapp.com/) to deliver push notifications from Home Assistant to your iOS device.
+
+Go to the [Prowl website](https://www.prowlapp.com/) and create a new API key.
+
+To add Prowl notifications to your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+notify:
+  - name: NOTIFIER_NAME
+    platform: prowl
+    api_key: ABCDEFGHJKLMNOPQRSTUVXYZ
+```
+
+Configuration variables:
+
+- **name** (*Optional*): Setting the optional parameter `name` allows multiple notifiers to be created. The default value is `notify`. The notifier will bind to the service `notify.NOTIFIER_NAME`.
+- **api_key** (*Required*): The Prowl API key to use.
+
+### {% linkable_title Prowl service data %}
+
+The following attributes can be placed `data` for extended functionality.
+
+| Service data attribute | Optional | Description |
+| ---------------------- | -------- | ----------- |
+| `priority`             |      yes | Priority level, for more info refer to the [Prowl API documentation](https://www.prowlapp.com/api.php#add). |
+
+To use notifications, please see the [getting started with automation page](/getting-started/automation/).