Slack has a great interface, easy setup, and you'll see Statuspage.io on the list of services that they integrate with.
First, you'll need to make sure you enable the ChatOps Add-on in Statuspage.
Next, you'll need to activate the Statuspage.io service in Slack.
Copy the Webhook key.
Once your key has been copied, head over to the ChatOps section of Statuspage and paste in your webhook URL.
Now add the channels you'd like Statuspage to post to.
Once you add your channels, you can select which types of events you want Statuspage to post in each Slack channel.