User can set notifications about step statuses changes.
Notifications can be set on process creation wizard or process settings.

Open process builder. Click menu "Actions" (1) and select  "Open Process Settings" (2) to open "Edit Process Settings" popup (3):

Select "Notifications Settings" tab (4) for configuring notifications:

• Chatter Notifications that will send to chatter feed and Assignee email (if configured)
• Object Notifications
• Slack Notifications that will be send to specified Slack channel ( available from version 5.0 ).

Turn on "Chatter Notifications" and/or "Object Notifications".

Check checkbox "Override Default Notification Message" to configure custom notification message.  Click arrow (5) to show formula builder for assistance.

Chatter notifications will be sent to users (assignees/admins) chatter feed (and email specified in user settings). Object notifications will be sent to the feed of object which record current process is started on:

Open "Slack Settings" picklist (6) to select Slack channel to send notifications to:

To make Slack notifications available you need to configure them once. Please refer to "Slack Settings" section of "Work-Relay Settings" article.