Add a screenshot in the main readme

[fzaninotto]

Hi guys,

And congrats, this looks very promising.

To boost engagement, I advise you to add a screenshot / screencast to the main site and the readme.

[kopax]

Hi and thanks. So far this is experimental, we expect to improve it slowly while increasing our backoffice code.
This is an adapter, even if sources of this package won't change (I assume), I agree we should update the readme/documentation asap. It may help find some contributors. I recommend you to watch ra-ui project (we do not use monorepo) for following changes.

• Our UI lib is: https://bootstrap-styled.github.io/ra-ui/
• A demo is available (based on the official): https://bootstrap-styled.github.io/ra-ui/demo

UI is not ready for distribution, we should also add a warning while it's under construction.

/cc @santaclauze