Document usage with Enzyme 3

Created by: gaearon

Our docs are outdated.

People are getting confused thinking they need to eject: https://github.com/facebookincubator/create-react-app/issues/3268.

Need to fix the rAF warning and then put something like https://github.com/facebookincubator/create-react-app/issues/3199#issuecomment-334801311 in the docs both here and in Enzyme.