Update readme

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Account for fb2 books without covers

lector/parsers/mobi.py

@@ -21,10 +21,13 @@ import os
 import sys
 import shutil
 import zipfile
+import logging
 
 from lector.readers.read_epub import EPUB
 import lector.KindleUnpack.kindleunpack as KindleUnpack
 
+logger = logging.getLogger(__name__)
+
 
 class ParseMOBI:
     def __init__(self, filename, temp_dir, file_md5):