docs: add marimo usage to README

[akshayka]

This PR updates the English-language README with how to use Pygwalker in marimo.

Note that marimo patches pygwalker.walk to use pygwalker.api.marimo.walk, so pygwalker.walk(df) works in marimo. This was added in version 0.9.15 of marimo: marimo-team/marimo#2798

[ObservedObserver]

Thank you for the patches. I'll merge the PR, but I want to keep the main README simple to avoid overwhelming readers with too much information. Our API and documentation follow a progressive design approach, providing only the essential details to start.

Since the official pygwalker documentation is not well-maintained, I'll merge this PR now. We'll update our official docs later, where Marimo will have its own dedicated page or section. After that, I'll create a simpler version of the main README.

[akshayka]

That makes sense to me. Thank you!