Add explicit license text

[csmith]

The README says the license is MIT, but it's useful to have the full text to reference as well.

This makes github recognise the license, and satisfies automated license scanners and pointy-haired bosses.

[heyLu]

Having the full license text would also help https://pkg.go.dev/github.com/litao91/goldmark-mathjax display the documentation, which it currently doesn't because it can't detect a license.