Document stories not showing up on storybook UI until device connects

[shilman]

Issue: N/A

What I did

Document wonky RN storybook behavior for first time users

How to test

N/A

[codecov]

Codecov Report

Merging #1221 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1221   +/-   ##
=======================================
  Coverage   13.72%   13.72%           
=======================================
  Files         207      207           
  Lines        4640     4640           
  Branches      520      514    -6     
=======================================
  Hits          637      637           
- Misses       3543     3550    +7     
+ Partials      460      453    -7

Impacted Files	Coverage Δ
...react-native/src/manager/components/PreviewHelp.js	0% <ø> (ø)	⬆️
addons/knobs/src/components/PropField.js	10.86% <0%> (ø)	⬆️
addons/knobs/src/components/types/Color.js	8% <0%> (ø)	⬆️
addons/knobs/src/components/types/Object.js	5.81% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/left_panel.js	20% <0%> (ø)	⬆️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/types/Number.js	8.06% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	0% <0%> (ø)	⬆️
addons/knobs/src/KnobStore.js	6.81% <0%> (ø)	⬆️
... and 17 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 467d671...eb15170. Read the comment docs.

[orta]

This makes sense 👍

[slorber]

@shilman what would be very great is to have a big green/red icon in the welcome page of http://localhost:7007/, just under the instructions, to tell the user weither or not the storybook has received an incoming connexion from device

[shilman]

@slorber that's a great idea. PR's welcome!!

[tmeasday]

LGTM