add version number

[maruilian11]

hi, recently we want to your lib on https://cdnjs.com, i think maybe this lib only has one version, can we regards it as v1.0.0 on cdnjs? or if you have free time, we hope you can add git tag for this version, just in case if there will be a later version, our git auto-update config can add it automatically. thank you very much!

https://github.com/cdnjs/cdnjs/issues/7125

[mhayes]

@rafibomb can we tag the current release as v1.0.0 here: https://github.com/zurb/twentytwenty/releases

Thanks!