title: PrimaryKeyRelatedField fields prints ids of foreign table but need values
body: <p>PrimaryKeyRelatedField fields prints ids of foreign table but api demands to return values. Is there any customized way to get the values</p>
  <pre><code>class Album(models.Model):
      album_name = models.CharField(max_length=100)
      artist = models.CharField(max_length=100)
  
  class Track(models.Model):
      album = models.ForeignKey(Album, related_name='tracks')
      order = models.IntegerField()
      title = models.CharField(max_length=100)
      duration = models.IntegerField()
      
          class Meta:
              unique_together = ('album', 'order')
              order_by = 'order'
  
  class AlbumSerializer(serializers.ModelSerializer):
      tracks = serializers.PrimaryKeyRelatedField(many=True, read_only=True)
  
      class Meta:
          model = Album
          fields = ('album_name', 'artist', 'tracks')
  </code></pre>
  <p>Results :</p>
  <pre><code>{
      'album_name': 'The Roots',
      'artist': 'Undun',
      'tracks': [
          89,
          90,
          91,
          ...
      ]
  }
  </code></pre>
  <p>Is there any way to get output in the below format:</p>
  <pre><code>{
          'album_name': 'The Roots',
          'artist': 'Undun',
          'tracks': [
              {'order': 1, 'title': 'Public Service Announcement', 'duration': 245},
              {'order': 2, 'title': 'What More Can I Say', 'duration': 264},
              {'order': 3, 'title': 'Encore', 'duration': 159},
          ]
      }
  </code></pre>