`url(r'^login/', 'django.contrib.auth.views.login'),`

<form method="post" action=".">
    {% csrf_token %}

    <p>
      <label for="id_username">Username:</label>
      {{ form.username }}
    </p>
    <p>
      <label for="id_password">Password:</label>
      {{ form.password }}
    </p>
    {% if next %}
      <input type="hidden" name="next" value="{{ next }}" />
    {% else %}
      <input type="hidden" name="next" value="/" />
    {% endif %}
    <input type="submit" value="Login"/>
  </form>

<form>
  <div class="form-group">
    {{ form.username }}
  </div>
  <div class="form-group">
    {{ form.password }}
  </div>
  <button type="submit" class="btn btn-default">Submit</button>
</form>

username = forms.CharField(widget=forms.TextInput(attrs={'placeholder': 'Username', 'class': 'form-input'}))

::-webkit-input-placeholder { color:#f00; }
::-moz-placeholder { color:#f00; } /* firefox 19+ */
:-ms-input-placeholder { color:#f00; } /* ie */
input:-moz-placeholder { color:#f00; }

This can be easily done in template using django package widget_tweaks

so your code looks like

<form>
  <div class="form-group">
    {% render_field form.username placeholder="<your text>" %}
  </div>
  <div class="form-group">
    {% render_field form.password placeholder="<your text>" %}
  </div>
  <button type="submit" class="btn btn-default">Submit</button>
</form>

for more refer the page https://pypi.python.org/pypi/django-widget-tweaks