Include macro instructions for Storyshots docs

[kylemh]

Issue:

Background

Setting up Storybook in CRA2 with Storyshots is currently impossible without ejecting and using a plugin, or inlining a function which could be made into a library.

#4479 seems to cover the pain well.

I've published a macro for CRA2 users to easily mock/polyfill require.context in a test environment, and updated the documentation for Storyshots pointing out how to do so.

How to test

• npx create-react-app test
• Setup Storybook in test
• Integrate Storyshots addon
• Follow my instructions (in this PR)

[codecov]

Codecov Report

Merging #5015 into next will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             next    #5015   +/-   ##
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  Coverage   34.85%   34.85%           
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  Files         579      579           
  Lines        7037     7037           
  Branches      943      943           
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  Hits         2453     2453           
  Misses       4081     4081           
  Partials      503      503

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[tmeasday]

LGTM!