Update Flow instructions

[gaearon]

Flow has recently changed its defaults to better fit Create React App.
The only remaining issue is the one with fbjs so I left it in the workaround doc.

Great work Flow team!

[denkristoffer]

Running the latest Flow just built from master my config still needs

[ignore]
<PROJECT_ROOT>/node_modules/fbjs/.*
<PROJECT_ROOT>/node_modules/react/node_modules/fbjs/.*

[options]
module.name_mapper='^\(.*\)\.css$' -> 'react-scripts/config/flow/css'

[gaearon]

What error do you get if you remove the CSS mapping? Can you publish a project reproducing this?

[denkristoffer]

I'm getting errors like ./App.css. Required module not found. I'll see if I can recreate the problem in a new project.

[gaearon]

If you can please file an issue and we'll take a look. Thanks.