Create-react-app: Document usage with Enzyme 3

[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Our docs are outdated.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.

[ryansully]

I just set up Enzyme 3 with React 16 for a project recently. I'll tackle this issue.