Updated addon knob readme.

[Gongreg]

Updated addon knob readme.

Includes info about #1039.

[codecov]

Codecov Report

Merging #1406 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1406   +/-   ##
=======================================
  Coverage   14.35%   14.35%           
=======================================
  Files         201      201           
  Lines        4612     4612           
  Branches      510      567   +57     
=======================================
  Hits          662      662           
+ Misses       3504     3470   -34     
- Partials      446      480   +34

Impacted Files	Coverage Δ
app/react/src/demo/Welcome.js	0% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	0% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-build.js	0% <0%> (ø)	⬆️
addons/info/src/components/PropTable.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/handle_keyevents.js	33.33% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/down_panel.js	23.52% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
app/react/src/server/build.js	0% <0%> (ø)	⬆️
... and 17 more

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[ndelangen]

Perfect, thanks!