Improve documentation about adding TypeScript support for the sx prop

[bartlangelaan]

• I have followed (at least) the PR section of the contributing guide.

This clarifies in the README.md that you can import SxProp from @pigment-css/react to add sx support to all HTML elements.

[lpicchi]

Thank you with this change I was able to fix my Typescript errors:

This is my augmentation file for reference:

// any file that is included in your tsconfig.json
import { SxProp } from "@pigment-css/react";
import type { ExtendTheme } from "@pigment-css/react/theme";

declare module "@pigment-css/react/theme" {
  // eslint-disable-next-line @typescript-eslint/no-empty-object-type
  interface ThemeTokens {}

  interface ThemeArgs {
    theme: ExtendTheme<{
      colorScheme: "light" | "dark";
      tokens: ThemeTokens;
    }>;
  }
}

declare global {
  namespace React {
    // eslint-disable-next-line @typescript-eslint/no-unused-vars
    interface HTMLAttributes<T> {
      sx?: SxProp;
    }
  }
}