Update README for current authentication methods
Update README for current authentication methods

- There is now the ability to use any of the AWS SDK
  DefaultAWSCredentialsProviderChain credential providers
- Avoid recommending the project.clj having refs to
  :env/aws_access_key_id since AWS_ACCESS_KEY_ID is automatically
  supported now

s3-wagon-private/s3-wagon-private,technomancy/s3-wagon-private