Installing groovy plugins in Rundeck is as easy as 123. Just grab the SlackNotificationPlugin.groovy and place it in the Rundeck plugin folder.
No restart required, a Rundeck plugin scan will be with you shortly.
Don't forget to check out the official guide.
Configuration of notifications is done at job level.
Once the plugin has been picked up by Rundeck, browse to any of your jobs and: Job -> Edit -> Send Notification -> Slack and fill out your details.
For the visual minded folks an image.
The notifier supports onStart, onFailure and onSuccess notifications.
After that it should be smooth sailing from there on out.
Please submit a merge request when you have a bug or feature we might all benefit from!
Working with IntelliJ? use: gradle idea