Add docs on how to pass authorization keys for bucket

[remorses]

The docs here and the readme are missing information on how to pass authorization keys for the bucket, or does it work the the SSH key from fly?

[efirs]

The docs has configure credentials section.
You can also find configure credentails in prebuilt-deb-and-rpm-packages section of README.
Let me know if you are looking for something else or need more info.