Update ARCHITECTURE.md

[erdos]

The idea for maintaining an ARCHITECTURE.md document recently came up on HN: https://news.ycombinator.com/item?id=39494925

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.12%. Comparing base (18d13f3) to head (f04786d). Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   93.10%   93.12%   +0.02%     
==========================================
  Files          29       30       +1     
  Lines        2059     2065       +6     
  Branches      104      104              
==========================================
+ Hits         1917     1923       +6     
  Misses         38       38              
  Partials      104      104              

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