fix: update info about runtime JSX rendering

[cassiozen]

When using the react prop, we suggest calling React components as functions when JSX transpilation isn't available (e.g., EmailTemplate({firstName: "John", product: "MyApp"})). However, this approach has several limitations:

• Components that use hooks may not work correctly when called directly as functions
• React 19 introduces changes in component types (specifically React.FC) that will trigger errors

Solution: Recommend using jsx from react/jsx-runtime instead. This function is what build tools like Babel actually use to transform JSX at compile time, making it the most reliable alternative when JSX syntax isn't available.

This change updates the README to recommend the jsx function approach.

[gabrielmfern]

LGTM!