Update README.md

[jtheletter]

Add warning for data loss.

All submissions

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

Adding/updating Application X Support

• This PR is only for one application
• It has been added to the list of supported applications in the README
• Changes have been added to the WIP section of the CHANGELOG
• Syncing does not break the application
• Syncing does not compete with any syncing functionality internal to the application
• The configuration syncs the minimal set of data
• No file specific to the local workstation is synced
• No sensitive data is synced

Improving the Mackup codebase

• My submission passes the tests
• I have linted the code locally prior to submission
• I have written new tests as applicable
• I have added an explanation of what the changes do

[chrisbloom7]

Better than no notice at all I suppose, but unfortunately doesn't do anything to warn off existing users who assume it would continue to work as expected.

[chrisbloom7]

Perhaps it could mention the workaround that some folks have had success with.

[spuder]

Please merge this.

It's gotten the attention of Reddit

https://www.reddit.com/r/macapps/s/psgVPRH1ir