Mention that .babelrc might be cached

[kevinweber]

Summary

A common issue for developers using Webpack 2 is Jest's caching of .babelrc. @cpojer mentioned it on Mar 5, 2016: #770 (comment)
This PR adds a short note explaining how to disable the cache.

Test plan

I copied the new markup into Dilinger's online markdown editor to ensure it is formatted correctly.

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[codecov-io]

Codecov Report

Merging #3545 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3545   +/-   ##
=======================================
  Coverage   62.31%   62.31%           
=======================================
  Files         181      181           
  Lines        6708     6708           
  Branches        6        6           
=======================================
  Hits         4180     4180           
  Misses       2525     2525           
  Partials        3        3

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