Document example of ember-cli-head integration

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_