Override configs for CRA 3 with craco to enable decorators

[dwaltrip]

Perhaps we should add craco to the boilerplates section for those who want to use mobx + decorators with CRA without ejecting?

Their recipe worked for me on the first try:
https://github.com/sharegate/craco/blob/master/recipes/use-mobx/craco.config.js

I tried using customize-cra first, before finding craco, but my limited attempt to get it working failed.

[hawkins]

Never heard of it, looks nifty though