Fix #620: documented how to create library ID

docs/importing-music.rst

@@ -26,6 +26,8 @@ to the ``/music`` directory on the container:
     docker-compose run --rm api python manage.py import_files $LIBRARY_ID "/music/**/*.ogg" --recursive --noinput
 
 .. note::
+    You'll have to create a library in the Web UI before to get your library ID. Simply visit
+    https://yourdomain/content/libraries/ to create one.
 
     Library IDs are available in library urls or sharing link. In this example:
     https://funkwhale.instance/content/libraries/769a2ae3-eb3d-4aff-9f94-2c4d80d5c2d1,