Update jest.md

[wyatt-troia]

Add note about skipping setting 'setupFilesAfterEnv' if using create-react-app. If you try to do so when using create-react-app, you get the following error:

(base) ➜  component-library git:(setup) ✗ npm run test

> component-library@0.1.0 test
> react-scripts test

We detected setupFilesAfterEnv in your package.json.

Remove it from Jest configuration, and put the initialization code in src/setupTests.js.
This file will be loaded automatically.

[ljharb]

Skipping this step means your enzyme adapter isn't configured. What's the workaround?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 8.60%. Comparing base (cdfb1c6) to head (6017756).
⚠️ Report is 100 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (cdfb1c6) and HEAD (6017756). Click for more details.

HEAD has 79 uploads less than BASE

Flag	BASE (cdfb1c6)	HEAD (6017756)
	100	21

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #2499       +/-   ##
==========================================
- Coverage   96.12%   8.60%   -87.53%     
==========================================
  Files          49      16       -33     
  Lines        4004    1802     -2202     
  Branches     1123     498      -625     
==========================================
- Hits         3849     155     -3694     
- Misses        155    1647     +1492     

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