Add macros section to readme.md

[tkh44]

These are the only 2 examples I know of at the moment. I'm sure @kentcdodds knows more.

[Haroenv]

You can find more if you go to https://yarnpkg.com/en/package/?q=babel-macro

[xtuc]

Thanks for the PR.

I'm not sure what we should do because there's already babel-macros and idx in other sections of the README. Should we merge them into a macro one?

[hzoo]

Yep I would move it into a section and explain what a macro is or link to the new post

[kentcdodds]

You'll find all the macros I'm aware of in the blog post :)