include LICENSE, README.md; use .tar.xz; use .zip on Windows

[jftuga]

This change to the .goreleaser.yml does the following:

• includes the dns binary as well as your README.md and LICENSE files in the Release section
• use .tar.xz instead of .tar.gz for better compression and smaller downloads
• use .zip on Windows since this file type can be opened by File Explorer without any additional third-party software