Since Userfeed is built 100% on Intercom, you can use Intercom's Zapier app to get notified in Slack when new feedback is created. Here's how:
Log in to your Zapier account (or sign up if you're a new user)
Click this link and follow our Zapier template: https://zapier.com/app/editor/template/37524
Note: This is not a native Userfeed/Slack integration so there are limitations. Only new feedback activity from users will show in the channel, not leads. Also, there isn't a direct link to the Userfeed post. These will be addressed in a native Slack integration in the future.