title: different fields for add and change pages in admin
body: <p>I have a django app with the following class in my admin.py:</p>
  
  <pre><code>class SoftwareVersionAdmin(ModelAdmin):
      fields = ("product", "version_number", "description",
        "media", "relative_url", "current_version")
      list_display = ["product", "version_number", "size",
        "current_version", "number_of_clients", "percent_of_clients"]
      list_display_links = ("version_number",)
      list_filter = ['product',]
  </code></pre>
  
  <p>I want to have these fileds for add page but different fields for change page. How can I do that?</p>