Bring Slack app docs to CE and add tiers

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+# GitLab Slack application **[FREE ONLY]**
+
+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 working
+with Slack on making this configurable for all GitLab installations, but there's
+no ETA.
+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.
+
+## Slack App Directory
+
+The simplest way to enable the GitLab Slack application for your workspace is to
+install the [GitLab application](https://slack-platform.slack.com/apps/A676ADMV5-gitlab) from
+the [Slack App Directory](https://slack.com/apps).
+
+Clicking install will take you to the
+[GitLab Slack application landing page](https://gitlab.com/profile/slack/edit)
+where you can select a project to enable the GitLab Slack application for.
+
+![GitLab Slack application landing page](img/gitlab_slack_app_landing_page.png)
+
+## Configuration
+
+Alternatively, you can configure the Slack application with a project's
+integration settings.
+
+Keep in mind that you need to have the appropriate permissions for your Slack
+team in order to be able to install a new application, read more in Slack's
+docs on [Adding an app to your team][slack-docs].
+
+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
+
+That's all! You can now start using the Slack slash commands.
+
+## Usage
+
+After confirming the installation, you, and everyone else in your Slack team,
+can use all the [slash commands].
+
+When you perform your first slash command you will be asked to authorize your
+Slack user on GitLab.com.
+
+The only difference with the [manually configurable Slack slash commands][slack-manual]
+is that all the commands should be prefixed with the `/gitlab` keyword.
+We are working on making this configurable in the future.
+
+For example, to show the issue number `1001` under the `gitlab-org/gitlab-ce`
+project, you would do:
+
+```
+/gitlab gitlab-org/gitlab-ce issue show 1001
+```
+
+[slack-docs]: https://get.slack.help/hc/en-us/articles/202035138-Adding-apps-to-your-team
+[slash commands]: ../../../integration/slash_commands.md
+[slack-manual]: slack_slash_commands.md