Figure out and document how to hook up components in WP

[oxyc]

• Polyfills only on old browsers
• Can we use <script type="module"> on modern on what?
• What happens with webpack and lazy loading etc?

[silentnoodlemaster]

adapted from the stencil documentation and tested in sage:

import { applyPolyfills, defineCustomElements } from 'genero-design-system/loader';

applyPolyfills().then(() => {
  defineCustomElements();
});

[oxyc]

Could you add this together with react usage to the readme?