title: Interactive drop down list django with option to add new entry
body: <p>I am relatively new to django.  I have made a simple app which enable a user to record   details regarding a run (Activity).  The user enters data such as: distance, time, route name, date ran..etc using a django ModelForm.</p>
  <p>I have a ModelForm which enables the logged on user to add an Activity to the model database.</p>
  <p>How do I get the form to add a drop down list of all the routes which are already in the model database AND add a new one if the Person is entering data about a route they haven't ran before.?</p>
  <pre><code>class Person(models.Model):
      user = models.OneToOneField(User, on_delete=models.CASCADE)
      fName = models.CharField(max_length =100)
      lName = models.CharField(max_length =100)
      weight = models.FloatField()
      sex = models.CharField(max_length=10)
  
      def __str__(self):
          return self.lName
  
  class Activity(models.Model):
      person = models.ForeignKey(Person, on_delete=models.CASCADE)
      length = models.FloatField()
      runTime = models.DurationField()
      route = models.CharField(max_length=100)
      ave = models.DurationField()
      dateRun = models.DateField()
      marathon = models.DurationField()
  
      def __str__(self):
          return self.route ```
  </code></pre>