Add jscodeshift codemod to README

[jayu]

Hey, I created a jscodeshift codemod for migration from template literal based class names concatenation to this library. It would be awesome to link it somewhere in the readme, then potentially more people could use it. I'm happy to open a PR if you don't mind

Here is the codemod https://github.com/jayu/template-literal-classnames-codemod