Update response model documentation to explain design choice

[JHSaunders]

Closes #101

[codecov]

Codecov Report

Merging #109 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #109   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         124    124           
  Lines        2987   2987           
=====================================
  Hits         2987   2987

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[codecov]

Codecov Report

Merging #109 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #109   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         124    125    +1     
  Lines        2987   3007   +20     
=====================================
+ Hits         2987   3007   +20

Impacted Files	Coverage Δ
tests/test_additional_properties.py	100% <0%> (ø)

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[tiangolo]

Great! Thank you. I updated it a bit and merged. 🌮 🎉