doc: s3 support - sync, backup, mount

[diepes]

Reading the repo readme wiki, it is not clear if the following can be achieved.

1. casync a local disk/fs to aws s3 bucket, as chunks and index.
2. later casync another backup to s3, sending only diff/different chunks ?
3. mount over https a remote s3 casync as a local filesystem ?

It seems all the components might be there.

[gyscos]

Projects like https://github.com/s3fs-fuse/s3fs-fuse allow transparently mounting a s3 bucket. Using the mounted bucket as target for the casync make command saves the chunks there, and on the next backup, only the different chunks (the "diff") will need to be uploaded again.

Mounting an archive is already in the docs: since you can access s3 files from simple HTTP requests, this example applies (just use your s3 address instead):

casync mount --seed=/home/lennart http://www.foobar.com/lennart.caidx /home/lennart2

[poettering]

support for this is not implemented right now, but implementing it should be relatively straight-forward. I have no experience with s3, but under the assumption uploading is also done with HTTP requests of some form it should be pretty easy to hack this up. All we need is a way to place files on the share in a way that returns an error if an object by the same name is already there, rather than overwrite it.

Patches for that very welcome!