docs: add instructions for using babel.config.js (#34)
docs: add instructions for using babel.config.js (#34)

Hi - I have been using babel-plugin-tester for a bit with my own plugins (LOVE IT!).  I discovered today that it already works with `babel.config.js`, but wanted to update the documentation for anyone else who runs into this problem.  The only changes are to README.md, no actual code changes.

Thanks!

PS I did not add myself to .all-contributorsrc because I haven't made any code changes, if you would prefer that I do this, please let me know.
babel-utils/babel-plugin-tester,babel-utils/babel-plugin-tester