I added an additional explanation about S3 tempral storage. When digdag tries to connect with S3 temporal storage, it can use some types of credential besides aws_access_key. Thus, I described about that on README.
I fixed the description of use_environment_file by removing an unnecessary statement.
This PR is related to https://github.com/treasure-data/digdag/pull/1759
aws_access_key. Thus, I described about that on README.use_environment_fileby removing an unnecessary statement.