Add documentation about use with S3

[zimeon]

See #25 comments for some recent demos. Would also be great to have some S3 tests built in but not sure how to provide something to test against.

[awoods]

As a part of this documentation, it will be helpful to know how to:

• provide S3 credentials
• provide non-AWS --endpoint-url

[IllyaMoskvin]

We can pass a non-AWS endpoint URL by adding an endpoint_url query string parameter to e.g. the objdir:

python ocfl-object.py extract --objver head --objdir "s3://BUCKET/1234/5678/87654321/?endpoint_url=https://example.com" --dstdir dest/87654321

Relevant code: https://github.com/zimeon/ocfl-py/blob/2613eef/ocfl/pyfs.py#L77