In importer.conf example, explain why the source DB can't be open read-only

importer.conf

@@ -49,6 +49,8 @@ database=neo
 # Example with FileStorage but this can be anything else.
 # ZEO is possible but less efficient: ZEO servers must be stopped
 # if NEO opens FileStorage DBs directly.
+# Note that NEO uses 'new_oid' method to get the last OID, that's why the
+# source DB can't be open read-only. NEO never modifies a FileStorage DB.
 storage=
   <filestorage>
     path /path/to/root.fs