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[bot]]

Codecov Report

Merging #2499 (6017756) into master (cdfb1c6) will decrease coverage by 87.52%. The diff coverage is n/a.

@@            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     

Impacted Files	Coverage Δ
packages/enzyme/src/ShallowWrapper.js	4.57% <0.00%> (-94.49%)	:arrow_down:
packages/enzyme/src/ReactWrapper.js	6.56% <0.00%> (-92.90%)	:arrow_down:
packages/enzyme/src/RSTTraversal.js	7.27% <0.00%> (-90.00%)	:arrow_down:
packages/enzyme/src/validateAdapter.js	10.00% <0.00%> (-90.00%)	:arrow_down:
packages/enzyme-shallow-equal/src/index.js	11.76% <0.00%> (-88.24%)	:arrow_down:
packages/enzyme/src/EnzymeAdapter.js	12.50% <0.00%> (-87.50%)	:arrow_down:
packages/enzyme/src/Utils.js	7.72% <0.00%> (-87.13%)	:arrow_down:
packages/enzyme/src/Debug.js	14.86% <0.00%> (-85.14%)	:arrow_down:
packages/enzyme/src/configuration.js	18.18% <0.00%> (-81.82%)	:arrow_down:
packages/enzyme/src/selectors.js	14.72% <0.00%> (-81.73%)	:arrow_down:
... and 37 more

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