Add notes about Jest to Readme

[timarney]

We need to document how to configure Jest

1). Add note .babelrc is needed for jest to use plugins used by rewires
2). Option to configure via package.json
3). Configuring via the export config (maybe when to use this vs package.json)

module.exports = {
  webpack: (config, env) => {
    config = rewireStyledComponents(config, env);
    return config;
  },
  jest: (config) => {
    ...your modifications...
    return config;
  }
}

Anyone want to tackle this?