This trivial patch adds a note about the minimum version of rclone
needed in order to be able to use multiple application keys with the b2
backend.
As Debian stable (amongst other distros) is shipping an older version,
users running rclone < 1.43 and reading about this feature in the online
docs may struggle to realise why they are not able to sync to b2 when
configured to use an application key other than the master one.
For reference: https://github.com/ncw/rclone/issues/2513