Add short description to addon-options readme

[danielduan]

Issue: These options are not clear on what exactly they do.

What I did

added a short description. any opinions on whether the options listed should show our defaults?

How to test

[shilman]

or null to disable?

[shilman]

Great addition!

[codecov]

Codecov Report

Merging #1566 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1566   +/-   ##
=======================================
  Coverage   20.43%   20.43%           
=======================================
  Files         241      241           
  Lines        5250     5250           
  Branches      647      650    +3     
=======================================
  Hits         1073     1073           
+ Misses       3697     3685   -12     
- Partials      480      492   +12

Impacted Files	Coverage Δ
addons/knobs/src/components/types/Boolean.js	11.62% <0%> (ø)	⬆️
addons/info/src/components/Props.js	37.2% <0%> (ø)	⬆️
addons/storyshots/src/storybook-channel-mock.js	0% <0%> (ø)	⬆️
addons/knobs/src/KnobManager.js	35.41% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	24.13% <0%> (ø)	⬆️
addons/knobs/src/components/types/Select.js	7.81% <0%> (ø)	⬆️
addons/info/src/components/Node.js	38.88% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	30% <0%> (ø)	⬆️
lib/ui/src/modules/api/configs/init_api.js	40.47% <0%> (ø)	⬆️
app/react-native/src/bin/storybook-start.js	0% <0%> (ø)	⬆️
... and 16 more

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