Fixed #15968 - Noted that readonly_fields are excluded from the ModelForm

    By default the admin shows all fields as editable. Any fields in this

    option (which should be a ``list`` or ``tuple``) will display its data

    as-is and non-editable. Note that when specifying :attr:`ModelAdmin.fields`

    or :attr:`ModelAdmin.fieldsets` the read-only fields must be present to be

    shown (they are ignored otherwise).

    as-is and non-editable; they are also excluded from the

    :class:`~django.forms.ModelForm` used for creating and editing. Note that

    when specifying :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets`

    the read-only fields must be present to be shown (they are ignored

    otherwise).

    If ``readonly_fields`` is used without defining explicit ordering through

    :attr:`ModelAdmin.fields` or :attr:`ModelAdmin.fieldsets` they will be