add note for pdftotext

doc/README.Converters

@@ -9,6 +9,9 @@ application/csv
 application/pdf
   pdftotext -nopgbrk %s - | sed -e 's/ [a-zA-Z0-9.]\{1\} / /g' -e 's/[0-9.]//g'
 
+	If pdftotext takes too long on large document you may want to pass parameter
+	-l to specify the last page to be converted
+
   mutool draw -F txt -q -N -o - %s 
 
 application/vnd.openxmlformats-officedocument.wordprocessingml.document