Merge branch 'zj-slack-docs' into 'master'

Slack docs [ci skip]

See merge request !8269

doc/integration/chat_commands.md

+# Chat Commands
+
+Chat commands allow user to perform common operations on GitLab right from there chat client.
+Right now both Mattermost and Slack are supported.
+
+## Available commands
+
+The trigger is configurable, but for the sake of this example, we'll use `/trigger`
+
+* `/trigger help` - Displays all available commands for this user
+* `/trigger issue new <title> <shift+return> <description>` - creates a new issue on the project
+* `/trigger issue show <id>` - Shows the issue with the given ID, if you've got access
+* `/trigger issue search <query>` - Shows a maximum of 5 items matching the query
+* `/trigger deploy <from> to <to>` - Deploy from an environment to another

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`, 
+all review the [full list of commands](../integrations/chat_commands.md).
+
+## Prerequisites
+
+A [team](https://get.slack.help/hc/en-us/articles/217608418-Creating-a-team) in Slack should be created beforehand, GitLab cannot 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!