Add warning to readme

[pxlrbt]

Hey,

since mackup seems to break on all new versions of macOS due to changes in symlinking, I think there should be a warning on the readme until the copy feature is shipped.

All submissions

• [x] I have followed the Contributing Guidelines
• [x] I have checked to ensure there aren't other open Pull Requests for the same update/change

[tslmy]

Thanks! Indeed, I noticed my computer freezing while using PyCharm after applying Mackup. I'm on 14.4 (23E214).

[obra]

As a macos user who was just about to install Mackup, this feels like it might be really good to get merged sooner rather than later.