Add a readme to react-scripts and create-react-app packages

[fson]

The create-react-app package on npm doesn't have a README associated with it: https://www.npmjs.com/package/create-react-app

The react-scripts package has the main README on npm, but I think it won't have it in the monorepo configuration unless we create one in that package folder.

I think we could keep publishing the default README to react-scripts by changing the release script so that it copies it over, or just make a shorter README for each package where we just explain what it is an link back to the main project.

[gaearon]

make a shorter README for each package where we just explain what it is an link back to the main project.

This makes sense to me and sounds fairly simple.

[gaearon]

Fixed by 92afcaf.