Fixed #24811 -- Added details on installing PostgreSQL extensions.

    2. Setup the hstore extension in PostgreSQL before the first ``CreateModel``

       or ``AddField`` operation by adding a migration with the

       :class:`~django.contrib.postgres.operations.HStoreExtension` operation.

       For example::

            from django.contrib.postgres.operations import HStoreExtension

            class Migration(migrations.Migration):

                ...

                operations = [

                    HStoreExtension(),

                    ...

                ]

       Creating the extension requires a database user with superuser

       privileges. If the Django database user doesn't have superuser

       privileges, you'll have to create the extension outside of Django

       migrations with a user that has the appropriate privileges. In that

       case, connect to your Django database and run the query

       ``CREATE EXTENSION IF NOT EXISTS 'hstore';``

    You'll see an error like ``can't adapt type 'dict'`` if you skip the first

    step, or ``type "hstore" does not exist`` if you skip the second.