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

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...