add a go.mod file so as to prepare for vgo; https://github.com/golang/vgo

But the main reason I added this PR is so as to ask the maintainer to properly semver tag this repository.
right now github.com/rs/xid has git tags likev1.0 v1.1
If you have github.com/rs/xid as a dependency on your app and you use vgo as dependency manager; it is generating a go.mod file like;

module github/komuw/myModule

require (
	github.com/rs/xid v0.0.0-20170604230408-02dd45c33376
	github.com/rs/zerolog v1.4.0
)

instead of picking xid tag v1.1

So my proposal is that, this project should add a new semver tag like; v1.1.0