Add a goreleaser configuration and workflow

[dominics]

Adds a .goreleaser.yml and a Github Actions workflow to do a release when a new tag is pushed. Fixes #204

The config was generated with goreleaser init, then modified to:

• turn on GO111MODULE
• point at cmd/desync as main
• ignore old arm64 versions on go 1.15 (see below)

This configuration is using the same Go version as the verify workflow (1.15), so windows/arm64 and darwin/arm64 platforms are ignored until we're using Go 1.16/1.17 (so, no Apple Silicon build yet). This ignore happens anyway in the latest versions of goreleaser, but wasn't happening on the slightly older version of goreleaser included in the action workflow (so, we'd fail like this)

This configuration runs a release workflow on tag, and produces releases that look something like this (but with a slightly nicer changelog once this squash-merges)

[folbricht]

Thank you. I think we should update go.mod to require Go 1.16, especially if that allows producing binaries for more platforms automatically. Would you like to make a PR for that?