title: Django, have model to be related to many instances of itself
body: <p>I have a model:</p>
  <pre><code>class Industry(models.Model):
      name = models.CharField(max_length=60, unique=True, null=False, blank=False)
  </code></pre>
  <p>I want it to have a <code>sub_industries</code> field that will hold all of its sub industries, which should be instances of <code>Industry</code> model.</p>
  <p>I cannot be <code>ManyToMany</code> because each industry has only one parent industry.</p>