Update README to describe public s3 bucket policy

The automatic setting of a public-read ACL for each artifact is removed in: https://github.com/spring-projects/aws-maven/commit/2ece0db9b0f381a8800441e9c64b8fcb72d0415e 

If somebody needs the old behaviour, needs to set a bucket policy. The provided script automates the process.
see issues:
- https://github.com/spring-projects/aws-maven/issues/6
- https://github.com/spring-projects/aws-maven/issues/8
- https://github.com/spring-projects/aws-maven/issues/19