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

[resolves #20]
Merge branch 'lalyos-patch-1'