Add a README.md file to the backup directory

[personalizedrefrigerator]

Summary

Adds a README.md file to the root backup directory that links to Simple Backup's README.

Example directory structure:

Example README content:

# Joplin Backup

This folder contains one or more backups of data from the Joplin note taking application. The most recent backup was created on 2/14/2024, 4:31:21 PM.

See the [Simple Backup documentation](https://joplinapp.org/plugins/plugin/io.github.jackgruber.backup/#restore) for information about how to restore from this backup.

Notes

• Does not commit changes to package.json
• Should adding a README file to the backup directory be a setting?

[personalizedrefrigerator]

Alternatively, to link to GitHub:

Suggested change

	"backupReadme": "# Joplin Backup\n\nThe backups in this folder were created with the Simple Backup plugin for Joplin. The most recent backup was created on %s.\n\nSee the [Simple Backup documentation](https://joplinapp.org/plugins/plugin/io.github.jackgruber.backup/#restore) for information about how to restore from this backup.",
	"backupReadme": "# Joplin Backup\n\nThe backups in this folder were created with the Simple Backup plugin for Joplin. The most recent backup was created on %s.\n\nSee the [Simple Backup documentation](https://github.com/JackGruber/joplin-plugin-backup?tab=readme-ov-file#restore) for information about how to restore from this backup.",

[JackGruber]

We should then also write a test for the revisions and the README.md location

[JackGruber]

I think we should use this.backupBasePath otherwise the README.md is in subfolders if there are several revisions.

[JackGruber]

Thanks, I have added the German translation and removed the timestamp from the README, as this information is not meaningful if several profiles are backuped.