README.md output path consistency

[supirman]

Please make the output path to be consistent in example, so it easy to follow for beginer like me.
It has 3 different value :

• public/react-toolbox in gif
• assets/react-toolbox in README config example
• src/toolbox in src/constants.js default setting

[javivelasco]

The gif is outdated but its not worthy to re-record it just for the output path so I've added a note before it's shown in the README telling that the current output path is src/toolbox. In the README example it's ok because we are setting in the package: "output": "assets/react-toolbox". If you omit the setting it defaults to src/toolbox