Document that --restrict-to-path is already taken care of in run.sh

[sowbug]

Thanks a bunch for making this image! I was going crazy trying to figure out why I couldn't add the --restrict-to-path magic to my client ssh keys. I was getting ERROR: /home/borg/.ssh/authorized_keys is not a public key file.. But once I looked at (https://github.com/Nold360/docker-borgserver/blob/master/data/run.sh#L12, I saw that you'd taken care of all this for us.

It would be nice to add language in the readme to reassure people that this is already done for them, and they don't need to mess around with custom ssh restrictions. Thanks again!