Move introduced-in text to correct place

The introduced-in line should not be buried in a note, but styled
according to the style guide.

doc/user/project/integrations/gitlab_slack_application.md

 # GitLab Slack application **(FREE ONLY)**
 
+> - Introduced in GitLab 9.4.
+> - Distributed to Slack App Directory in GitLab 10.2.
+
 NOTE: **Note:**
 The GitLab Slack application is only configurable for GitLab.com. It will **not**
 work for on-premises installations where you can configure the
 [Slack slash commands](slack_slash_commands.md) service instead. We're planning
 to make this configurable for all GitLab installations, but there's
 no ETA - see [#28164](https://gitlab.com/gitlab-org/gitlab/issues/28164).
-It was first introduced in GitLab 9.4 and distributed to Slack App Directory in
-GitLab 10.2.
 
 Slack provides a native application which you can enable via your project's
 integrations on GitLab.com.
@@ -35,12 +36,30 @@ docs on [Adding an app to your team](https://slack.com/help/articles/202035138).
 
 To enable GitLab's service for your Slack team:
 
-1. Go to your project's **Settings > Integration > Slack application** (only
-   visible on GitLab.com)
-1. Click the "Add to Slack" button
+1. Go to your project's **{settings}** **Settings > Integration > Slack application** (only
+   visible on GitLab.com).
+1. Click **Add to Slack**.
 
 That's all! You can now start using the Slack slash commands.
 
+## Create a project alias for Slack
+
+To create a project alias on GitLab.com for Slack integration:
+
+1. Go to your project's home page.
+1. Navigate to **{settings}** **Settings > Integrations** (only visible on GitLab.com)
+1. On the **Integrations** page, click **Slack application**.
+1. The current **Project Alias**, if any, is displayed. To edit this value,
+   click **Edit**.
+1. Enter your desired alias, and click **Save changes**.
+
+Some Slack commands require a project alias, and fail with the following error
+if the project alias is incorrect or missing from the command:
+
+```plaintext
+GitLab error: project or alias not found
+```
+
 ## Usage
 
 After confirming the installation, you, and everyone else in your Slack team,