docs: added nuxt@>3 instructions

[shorwood]

If it is of any help, here is a PR extending the Readme for anyone that might have want to use this plugin with nuxt@3

Added part

If using nuxt@>3 or nuxt-edge, you can leverage the virtual file system to import the generated CSS.

// nuxt.config.ts
import { defineNuxtConfig } from 'nuxt'
import Fonts from 'vite-plugin-fonts'

export default defineNuxtConfig({
  css: [
    'virtual:fonts.css',
  ],
  vite: {
    plugins: [
      Fonts({ /** vite-plugin-fonts config */}),
    ],
  },
})