Document example of ember-cli-head integration

[raido]

Integration with ember-cli-head can be done with v6.1.0 like this:

// app/services/page-title.js

import EmberPageTitleService from 'ember-page-title/services/page-title';

export default class PageTitleService extends EmberPageTitleService {
  @service headData;

  titleDidUpdate(title) {
    this.headData.ogTitle = title;
  }
}

Would be nice to include this into README / public docs app.

Originally posted by @raido in https://github.com/ember-cli/ember-page-title/issues/201#issuecomment-761081734

[raido]

One more example - https://github.com/ember-learn/ember-website/pull/719/files