Add documentation for Rocket.Chat notifications. (#3424)

* Add documentation for Rocket.Chat notifications.

* Update 'ha_release'

source/_components/notify.rocketchat.markdown

@@ -0,0 +1,56 @@
+---
+layout: page
+title: "Rocket.Chat"
+description: "Instructions on how to add Rocket.Chat notifications to Home Assistant."
+date: 2017-09-23 20:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: rocketchat.png
+ha_category: Notifications
+ha_release: 0.56
+---
+
+
+The `rocketchat` platform allows you to send messages to your [Rocket.Chat](https://rocket.chat/) instance from Home Assistant.
+
+To add Rocket.Chat to your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+notify:
+  - platform: rocketchat
+    name: NOTIFIER_NAME
+    url: https://rocketchat.example.com
+    username: USERNAME
+    password: PASSWORD
+    room: my-awesome-room
+```
+
+- **name** (*Optional*): Name displayed in the frontend. The notifier will bind to the service `notify.NOTIFIER_NAME`.
+- **url** (*Required*): The URL of your Rocket.Chat instance.
+- **username** (*Required*): The Rocket.Chat username.
+- **password** (*Required*): The Rocker.Chat password.
+- **room** (*Required*): The chat room name to send messages to.
+
+### {% linkable_title script.yaml example %}
+
+```yaml
+rocketchat_notification:
+  sequence:
+  - service: notify.NOTIFIER_NAME
+    data:
+      message: "Message to Rocket.Chat from Home Assistant!"
+      data:
+        emoji: ":smirk:"
+```
+
+#### {% linkable_title Message variables %}
+
+- **message** (*Required*): Message to be displayed.
+- **data** (*Optional*): Dictionary containing any of the variables defined in the
+           [Rocket.Chat docs](https://rocket.chat/docs/developer-guides/rest-api/chat/postmessage#message-object-example)
+
+To use notifications, please see the [getting started with automation page](/getting-started/automation/).
+