[docs] update file layout & usage section

[hwine]

Roughly (my interpretation):

• __init__.py is just to mark directory as a package
• client.py has all the code related to communication with the service
• helpers.py has all the code related to simplifying tests (e.g. operations on data structures returned from service)

But I see that differs a bit from the design docs.

Again, this isn't a blocker for this commit, but it does look like the design & file layout needs a refresh. I'll open a separate issue.

_Originally posted by @hwine in https://github.com/mozilla/frost/pull/436#discussion_r562192928_

[hwine]

Current docs:

• don't define purpose of client.py
• don't mention usage of __init__.py
• don't show nested conftest.py files

[ajvb]

From #436 - I'd agree with the assumption you stated about "client.py has all the code related to communication with the service".