Update README.md to clarify the limitations of the plugin

[andrebiegel]

Adding more Information about what is going to be backuped an d what not. Here i stamples upon the server.keys issue in an migrations use case. Which points me to the thing that migration doesnt seem to be a valid use case for the plugin. Unfortunately i thought that a backup plugin is able to do it. So i made this pull request to slightly clarify which features this has.

Testing done

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• [ ] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
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• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue