title: How to add fields in the serializer?
body: <p>MODELS:</p>
  
  <pre><code>class Organization(models.Model):
      title = models.CharField(max_length=100)
      user = models.ForeignKey(User)
  
  class Person (models.Model):
      name = models.CharField(max_length=30)
      surname = models.CharField(max_length=30, blank=True, null=True)
      card_tag = models.CharField(max_length=100, unique=True)
      extra = models.TextField()
      organization = models.ForeignKey(Organization)
  </code></pre>
  
  <p>SERIALIZER:</p>
  
  <pre><code>class PersonSerializer(serializers.ModelSerializer):
      class Meta:
          model = Person
          fields = ['name', 'surname', 'card_tag', 'extra']
  </code></pre>
  
  <p>QUESTION:</p>
  
  <p>When I do request with POST method I transmit (name, surname, card_tag, extra) in JSON format and token of User in header.</p>
  
  <p>So how can I:<br>
  1) get 'organization' by user (token)</p>
  
  <p>2) and add 'organization' to the PersonSerializer when it will save given data to the database </p>
  
  <p>Thanks!!!</p>