Slack docs [ci skip]

doc/project_services/slack_slash_commands.md

+# Slack slash commands
+
+> Introduced in GitLab 8.15
+
+Slack commands give users an extra interface to perform common operations
+from the chat environment. This allows one to, for example, create an issue as
+soon as the idea was discussed in chat.
+For all available commands try the help subcommand, for example: `/gitlab help`
+
+## Prerequisites
+
+A [team](https://get.slack.help/hc/en-us/articles/217608418-Creating-a-team) in Slack should be create beforehand, GitLab can not create it for you.
+
+## Configuration
+
+First, navigate to the Slack Slash commands service page, found at your project's
+**Settings** > **Services**, and you find the instructions there:
+
+   ![Slack setup instructions](img/slack_setup.png)
+
+Once you've followed the instructions, mark the service as active and insert the token
+you've received from Slack. After saving the service you are good to go!