[1.8.x] Fixed #24811 -- Added details on installing PostgreSQL extensions.

Backport of 9ef2615d from master

docs/ref/contrib/postgres/fields.txt

@@ -257,6 +257,24 @@ HStoreField
     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.