Update README to point to `react-app-rewired` full installation

[vaughnkoch]

First of all, thanks for this package!

The current README tells you to install customize-cra and react-app-rewired with this command:

yarn add customize-cra react-app-rewired --dev

Unfortunately, the readme doesn't mention that you have to update your package.json to use react-app-rewired scripts. I spent several hours trying to debug why my webpack aliases weren't working, and it turned out that my app wasn't even using config-overrides.js. My suggestion would be to tell people to install react-app-rewired by following the install instructions for that package, instead of running that single installation line.

Also, it might help to suggest putting a log line in config-overrides.js just to show the user that it's getting executed.