Update README.md

Author: mxamber

README.md

@@ -19,3 +19,5 @@
 `bmpurge path/to/file -after "01 Jan 2018 12:00"` will erase all bookmarks created after 01 Jan 2018, 12:00.
 
 `bmpurge path/to/file -before "31 June 2018 22:00" -after "01 Jan 2018 12:00"` will erase all bookmarks created between 01 Jan 2018, 12:00 and 31 June 2018, 22:00.
+
+`bmpurge path/to/file -before "01 Jan 2018 12:00" -after "31 June 2018 22:00"` will erase all bookmarks *except* ones created between 01 Jan 2018, 12:00 and 31 June 2018, 22:00.