Add details for Tailwind v4 support

Tailwind v4 is now much easier to customize with its css-first configuration and automatic value detection. In this pull request, I simply suggest the addition of a specific section in the readme to explain to users, possibly coming here to upgrade a legacy project, how to add equivalent utilities by hand.

Here's a playground link displaying the result and showing that it works. https://play.tailwindcss.com/lC8MBCSicQ?file=css