Add integration with goreleaser

[fsouza]

This way we can get nice release notes in the GitHub release pages.

Some examples:

• s3-upload-proxy: https://github.com/fsouza/s3-upload-proxy/releases
• fake-gcs-server: https://github.com/fsouza/fake-gcs-server/releases
• tsuru: https://github.com/tsuru/tsuru/releases
• goreleaser itself: https://github.com/goreleaser/goreleaser/releases

The Drone integration would look just like s3-upload-proxy/fake-gcs-server: https://github.com/fsouza/fake-gcs-server/blob/74263482429995ad51c5974cb80cf8aa11b0917f/drone/.drone.jsonnet#L36-L52