update README to include detail about new bucket encryption

[stavxyz]

By default, bucket encryption was added in #7

THANKS @jaustinpage !

Let's add a little bit more information for humans in the README about rationale. Perhaps this could go into a "details about best practices" section. Details and features like this are why this project exist. It would be easy enough to quickly provision an s3 bucket for tf state without using terraform, but a module like this one allows a truly repeatable best practice setup.