Update readme

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Account for fb2 books without covers

lector/parsers/pdf.py

@@ -20,12 +20,15 @@
 
 import io
 import os
+import logging
 
 from PyQt5 import QtCore
 from bs4 import BeautifulSoup
 
 import popplerqt5
 
+logger = logging.getLogger(__name__)
+
 
 class ParsePDF:
     def __init__(self, filename, *args):