Add file.write and YAML information - Slack integration (home-assista…

…nt#12210)

* Add file.write and YAML information

* Fixed data and file order in YAML example.

source/_integrations/slack.markdown

@@ -15,9 +15,9 @@ The `slack` platform allows you to deliver notifications from Home Assistant to
 
 1. Create a [new app](https://api.slack.com/apps) under your Slack.com account
 2. Click the `OAuth & Permissions` link in the sidebar, under the Features heading
-2. In the Scopes section, add the `chat:write:user` scope, `Send messages as user`
-3. Scroll up to `OAuth Tokens & Redirect URLs` and click `Install App`
-4. Copy your `OAuth Access Token` and put that key into your `configuration.yaml` file -- see below
+3. In the Scopes section, add the `chat:write:user` scope, `Send messages as user`. If you wish to also be able to include files, you will need to include `files:write:user`. If you get a `missing_scope` error when trying to send a message, check these permissions.
+4. Scroll up to `OAuth Tokens & Redirect URLs` and click `Install App`
+5. Copy your `OAuth Access Token` and put that key into your `configuration.yaml` file -- see below
 
 <div class='note'>
 
@@ -132,6 +132,19 @@ Example for posting formatted attachment:
 }
 ```
 
+You can also use YAML to send messages from your automations
+
+```yaml
+
+    - service: notify.slack
+      data:
+        message: "Latest notification"
+        title: "Latest image"
+        target: ["#home-assistant"]
+        data:
+          file:
+            path: "/myfile.jpg"
+```
 Please note that both `message` is a required key, but is always shown, so use an empty (`""`) string for `message` if you don't want the extra text.
 
 To use notifications, please see the [getting started with automation page](/getting-started/automation/).