Added help text to the react-native preview panel

[matt-oakes]

What I did

I have added some help text into the preview panel for React Native projects. I took the text from the README and added it into the blank preview panel to give hints into using the tool if the user opens the browser UI first.

Currently, it's ugly but functional:

How to test

Open the browser UI for a React Native project and look in the preview panel.

[codecov]

Codecov Report

Merging #1142 into master will decrease coverage by 0.08%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1142      +/-   ##
==========================================
- Coverage   12.84%   12.75%   -0.09%     
==========================================
  Files         198      199       +1     
  Lines        4540     4571      +31     
  Branches      718      724       +6     
==========================================
  Hits          583      583              
- Misses       3322     3347      +25     
- Partials      635      641       +6

Impacted Files	Coverage Δ
app/react-native/src/manager/provider.js	0% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)

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[shilman]

@matt-oakes Thanks for this wonderful and useful idea. It will be a huge improvement for the RN experience!

My one request is to make it look a little more Storybook-like before we merge. Borrowing a few lines from the cli's Welcome should do the trick:

https://github.com/storybooks/storybook/blob/master/lib/cli/generators/WEBPACK_REACT/template/stories/Welcome.js

[matt-oakes]

@shilman Ahhh! As I've only used the React Native version I hadn't seen that screen before.

I have borrowed the styling and taking the first paragraph and combined it with the device running instructions. It now looks like this:

Let me know if you want me to make any more changes.

[shilman]

@matt-oakes Beautiful!!!

[matt-oakes]

@shilman I think beautiful might be overstating it a bit, but thank you 😉

[shilman]

@matt-oakes "relatively beautiful" then? anyway, it should make a night-and-day difference to the onboarding experience for RN. i'm thrilled at this addition.

[matt-oakes]

@shilman Yep, hopefully it will!