Add comment for reading single files from URL for validation.

validation/validate-mmcif-file.py

@@ -297,6 +297,9 @@ def _get_associated_files(model_cif_file, assoc_dir):
                 )
             if row[idxs["file_format"]] != "cif":
                 continue
+            # this should be easy to make reading URLs, using
+            # _get_assoc_obj(row[idxs["file_url"]], assoc_dir) but for now
+            # I have not seen the use case (no web server at hand for testing)
             data = _read_mmcif(os.path.join(assoc_dir, row[idxs["file_url"]]))
             assoc_files.append((data, row[idxs["entry_id"]]))
         # make sure entry_id is matching in associated file!