Fixed #20493 -- Added a warning that objects may not be picklable across Django versions

When you are sufficiently confident your app works with the new version of

Django, you're ready to go ahead and :doc:`deploy </howto/deployment/index>`

your upgraded Django project.

If you are using caching provided by Django, you should consider clearing your

cache after upgrading. Otherwise you may run into problems, for example, if you

are caching pickled objects as these objects are not guaranteed to be

pickle-compatible across Django versions. A past instance of incompatibility

was caching pickled :class:`~django.http.HttpResponse` objects, either

directly or indirectly via the :func:`~django.views.decorators.cache.cache_page`

decorator.