add mod file and bump major version

[urandom2]

• continuation of module change from #52
• since your release tags are v2+ simply adding a go.mod file will cause v2.0.0+incompatible versions in consumers
  • the go authors suggest bumping the major version to resolve this: https://github.com/golang/go/wiki/Modules#incrementing-the-major-version-when-first-adopting-modules-with-v2-packages
  • after merge you should tag this repo with v3.0.0
  • you could migrate to v2.1.0, but this could cause consumer breakage
  • this uses the major branch paradigm, but you could leverage major subdirectory instead: https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher

[peterbourgon]

Does the lack of a go.mod cause concrete problems in downstream dependencies?

[urandom2]

While there is currently no breakage from the lack of a go.mod file, not having indirect dependencies tracked means that consumers have to track them directly:

module github.com/user/repo

go 1.12

require (
        github.com/google/btree v1.0.0 // indirect
        github.com/peterbourgon/diskv v2.0.1+incompatible
)

[metalmatze]

I'd appreciate having the mod file. Thanks.