Update import command

arkindex/documents/management/commands/import_manifest.py

 #!/usr/bin/env python3
 from django.core.management.base import BaseCommand, CommandError
 from arkindex.images.models import ImageServer
-from arkindex.documents.importer import LocalManifestsImporter
+from arkindex.documents.importer import LocalManifestsImporter, URLManifestsImporter
 import logging
 
 logging.basicConfig(
@@ -16,7 +16,7 @@ class Command(BaseCommand):
     def add_arguments(self, parser):
         parser.add_argument(
             'manifest_folder',
-            help='Folder to recursively search for IIIF manifests',
+            help='Folder to recursively search for IIIF manifests, or a URL pointing to a manifest',
             default='.'
         )
         parser.add_argument(