Document migrating the database.

docs/dev/02_install_apps.rst

@@ -16,3 +16,12 @@ Installing apps into the existing virtual environment is a bit awkward::
 This will install the Exlibris app (library management) app by using a
 shell for first ``cd``'ing into the app directory and then using
 poetry to install the app.
+
+
+Migrate the database
+--------------------
+
+After installing or updating any apps, the database must be updated as
+well by running Django's ``migrate`` command::
+
+  poetry run ./manage.py migrate