Update readme.md for use with Webpack 5

[dbuchet]

[lovasoa]

Maybe you could update the code itself to use the latest react and webpack ?

https://github.com/sql-js/react-sqljs-demo/blob/master/package.json

[dbuchet]

I've started with that, but I think many people are still with Webpack 4. So i've just updated the readme. Tell me what you prefer :)

[lovasoa]

I think we can update the code to the latest versions, and just leave a link in the readme to the version using webpack 4.

I tagged the current state of the repo as v1, so you can link to https://github.com/sql-js/react-sqljs-demo/tree/v1