Document Babel macros support and common uses

Created by: gaearon

Need to keep track:

• Relay https://github.com/facebook/relay/pull/2171
• Apollo https://github.com/facebookincubator/create-react-app/issues/3856#issuecomment-358696916
• Css-in-js like Emotion
• Maybe some i18n libs?
• SVG https://github.com/facebookincubator/create-react-app/issues/3856#issuecomment-358696916
• Maybe make one for idx? https://github.com/dralletje/idx.macro