Useful info about 'django-sortedm2m' package

With adding package `django-sortedm2m` in[ requirements.in](https://github.com/django-cms/djangocms-template/blob/master/backend/requirements.in#L44) there is a next comment:
> M2M drag and drop sorting for admin, don't use django-admin-sortable2 for M2M

Could we get a bit more details why we don't need to use `django-admin-sortable2`?
