Improve doc about passing AWS credentials

[demeringo]

Problem

Documentation is unclear about how to pass aws credentials when use in CLI.
Now only exporting the aws profile through env var (for Linux) is explained... and not in all sections.

See issue #76

Solution

Improve documentation. Describe and test various ways to pass AWS credentials for Unixes and Windows.

Alternatives

Additional context or elements

• https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-envvars.html
• https://docs.aws.amazon.com/powershell/latest/userguide/specifying-your-aws-credentials.html