Update CONTRIBUTING.md

[gmlewis]

@alexandear - what do you think about this paragraph in CONTRIBUTING.md?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.10%. Comparing base (2f8c45f) to head (d93bdce).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3736   +/-   ##
=======================================
  Coverage   91.10%   91.10%           
=======================================
  Files         187      187           
  Lines       16662    16662           
=======================================
  Hits        15180    15180           
  Misses       1295     1295           
  Partials      187      187           

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