Here are the instructions for setting up Slack as a Destination.
Click the "Slack" button under the Destinations tab, and type in a descriptive name. You’ll then see a URL for a RSS feed that you will copy and use in Slack.
Then go to Slack into the Apps section and add RSS feeds.
Copy and paste the RSS feed URL in and have it sent to a channel or to a specific user. You can then go into Workflows to assign content to your Slack RSS feed. Note: there may be a delay the first time you set it up, but results will start flowing after a short while.