Add state-setting FAQ

[shilman]

Issue: N/A

What I did

Document a pattern for setting react state in stories

How to test

N/A

[codecov]

Codecov Report

Merging #1559 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1559   +/-   ##
=======================================
  Coverage   20.46%   20.46%           
=======================================
  Files         241      241           
  Lines        5243     5243           
  Branches      645      648    +3     
=======================================
  Hits         1073     1073           
+ Misses       3673     3670    -3     
- Partials      497      500    +3

Impacted Files	Coverage Δ
addons/knobs/src/components/PropForm.js	8.51% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-build.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_routing.js	28.94% <0%> (ø)	⬆️
lib/ui/src/modules/api/actions/api.js	49.42% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
.../src/manager/containers/CommentsPanel/dataStore.js	34.78% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.81% <0%> (ø)	⬆️
addons/knobs/src/components/types/Object.js	5.81% <0%> (ø)	⬆️
... and 12 more

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