You can specify hooks to trigger certain actions upon certain events. To do so, head your App Settings page and find the Deploy hooks section.
You can post notifications to your slack channel by specifying a webhook. Currently there are three triggers: When a deployment has started, when a deployment has ended and when a deployment has been published. You can specify one, or more at the same time. To learn more about creating webhooks, you can read Slack's official documentation from here.