The source can found here on github.
The first release can be directly downloaded from the releases page.
So what does this module do?
It adds an extra notification action on documents. If a user clicks on this action, he/she is stored as extra metadata on the document.
When the document has been revised (new version), then all the users who wanted to be notified will be send an email that an update has been made to the document with the direct links in Alfresco.