start at updating docs for extra_files

[lwasser]

closes #178
This pr updates the documentation surrounding extra_files and updating the readme.

We have not merged #471 yet so i want to wait to address docs for that until we have merged it. Perhaps it would make sense to make that change within that open PR?

@kcranston would love you to review the language here to ensure it looks ok.

[codecov]

Codecov Report

Merging #484 (9a174a0) into main (d50e0a3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #484   +/-   ##
=======================================
  Coverage   60.47%   60.47%           
=======================================
  Files          20       20           
  Lines        1169     1169           
=======================================
  Hits          707      707           
  Misses        462      462           

Flag	Coverage Δ
unittests	60.47% <ø> (ø)

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