Add ability to subscribe to shared thing changes (folder or file)

I'd like to share a folder and let everyone subscribe to any changes in the folder or a file. For example, I shared resume folder to a few HRs and I want them to be able to subscribe via their email to be notified on any changes in this directory (like the cv.pdf has been modified, or new files added or so). Or another example, - I have shared "useful stuff" directory with my colleagues and friends and they are very interested in its contents. Now all of them have downloaded the whole directory and I need to tell them whenever I have added something new so they could find and download it again. I'd like to not spend my time on it and let them just subscribe via their email to any changes of what they are interested in.

Start with the issue description and trace the existing shared-folder and file-change notification flows. Define how email subscriptions apply to shared folders and individual files, including additions and modifications, and verify that subscribers receive notifications for the changes they requested.