Document how to install bundled apps.

docs/dev/02_install_apps.rst

@@ -8,3 +8,11 @@ Officially bundlede apps are available in the ``apps/official/``
 sub-folder as Git submodules. If you followed the documentation, they
 will already be checked out in the version required for the bundle you
 are running.
+
+Installing apps into the existing virtual environment is a bit awkward::
+
+  poetry run sh -c "cd apps/official/BiscuIT-App-Exlibris; poetry install"
+
+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.