Clarify where to export messenger data to, and fix export.py to pkl error

[aguldbrandsen]

I mistakenly exported my Facebook messenger data content to the messenger folder, instead of the entire messages folder i had unzipped from the Facebook download file. I saw in an other thread, that others had also done this. The readme could be changed so this would be avoided.

If i run export.py -format pkl then i get an error saying:

Traceback (most recent call last):
  File "export.py", line 59, in <module>
    main()
  File "export.py", line 52, in main
    with open(f_name, 'wb', encoding="utf8") as f:
ValueError: binary mode doesn't take an encoding argument

The error can be avoided by not setting encoding in the open() function.