todo: update docs for aws

[kurktchiev]

Update the documentation for AWS to state that we expect you to pass in your ~/.aws/credentials file or a file in that format to porter

[kurktchiev]

importantly, the profile must be [default]

if you have multiple profiles in your credentials file, it'll use the [default] profile

and if you don't have a [default] profile, you'll see the error (static credentials not found)