add more to file writing section

[natasha-aleksandrova]

ended up also adding Convert your main donor data structure to be a dict. - I still get occasional students that don't change main data structure to be a dictionary.

the rest are updates for file writing:

• mentioned hard coded paths - I get a ton of those
• tempfile - I think a cool option to learn and in general a good library to be aware of

[natasha-aleksandrova]

@PythonCHB flagging as a reminder 🙂

[PythonCHB]

LGTM -- And we're getting review on the mailroom assignment in another PR as well, so I'll merge and we can move on.

-CHB