How to expose reverse entity references?

Since Drupal 8 doesn't support bidirectional relationship, we need to query manually the reverse entity.

Let's say we have 2 entities with a many to many relationship:

We can add a field resolver to get all books from an author, like explained in [the doc](https://drupal-graphql.gitbook.io/graphql/v/8.x-4.x/queries/references#adding-resolvers):

```
$registry->addFieldResolver('Author', 'books',
$builder->compose(
$builder->produce('entity_reference')
->map('entity', $builder->fromParent())
->map('field', $builder->fromValue('field_books'))
)
);
```

But how can we get all authors from a book?