Document React 16.3 dependency

[shilman]

Issue: N/A

What I did

• Added migration docs
• Added a peer dep since Emotion doesn't seem to catch this

NOTE: I'm going to take a full pass at MIGRATION.md soon, just gathering up all the bullet points first.

[tmeasday]

Can we put the exact error message in here? Could help people googling

[tmeasday]

core.browser.esm.js:15 Uncaught TypeError: Object(...) is not a function
    at Module../node_modules/@emotion/core/dist/core.browser.esm.js (core.browser.esm.js:15)
    at __webpack_require__ (bootstrap:724)
    at fn (bootstrap:101)
    at Module../node_modules/@emotion/styled-base/dist/styled-base.browser.esm.js (styled-base.browser.esm.js:1)
    at __webpack_require__ (bootstrap:724)
    at fn (bootstrap:101)
    at Module../node_modules/@emotion/styled/dist/styled.esm.js (styled.esm.js:1)
    at __webpack_require__ (bootstrap:724)
    at fn (bootstrap:101)
    at Object../node_modules/@storybook/components/dist/navigation/MenuLink.js (MenuLink.js:12)

[codecov]

Codecov Report

Merging #4458 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4458   +/-   ##
=======================================
  Coverage   35.96%   35.96%           
=======================================
  Files         555      555           
  Lines        6653     6653           
  Branches      871      871           
=======================================
  Hits         2393     2393           
  Misses       3813     3813           
  Partials      447      447

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