Update README to note requires Django 1.8+
Update README to note requires Django 1.8+

jnns/django-import-export,django-import-export/django-import-export,daniell/django-import-export,jnns/django-import-export,jnns/django-import-export,bmihelac/django-import-export,django-import-export/django-import-export,daniell/django-import-export,bmihelac/django-import-export,daniell/django-import-export,django-import-export/django-import-export,bmihelac/django-import-export,jnns/django-import-export,django-import-export/django-import-export,bmihelac/django-import-export,daniell/django-import-export