[1.6.x] Added release note about strip_tags improvement

  :class:`~django.forms.IntegerField` subclasses from being overwritten by the

  code in their ``__init__`` method

  (`#22245 <http://code.djangoproject.com/ticket/22245>`_).

* Improved :func:`~django.utils.html.strip_tags` accuracy (but it still cannot

  guarantee an HTML-safe result, as stated in the documentation).