Add go.mod

[karelbilek]

This is strictly speaking not necessary, as there is no dependency, but it's helping some IDEs and gopls.

[lordofscripts]

I didn't know it isn't necessary, locally I can't compile without a go.mod. I hope the author reacts and merges this SOON.

[karelbilek]

Ah you're right.

I meant it's not necessary when this is a dependency, not when you try to build it locally. But you're right.