title: How to add placeholder to forms of Django-Registration
body: <p>I am using django-registration for my project. in my <code>registration_form.html</code> file:</p>
  
  <pre><code>{{form.username}}
  {{form.email}}
  //other fields
  </code></pre>
  
  <p>And I want to set placeholders for each field. But this is a kind of built-in app. so I need to find the way for editing these fields from my main app.</p>
  
  <p>I don't want to change source of django-registration.</p>