Add documentation for Dialog Component

[Josh Weinstein]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[ ] Feature request
[x] Documentation issue or request
[ ] Other... Please describe:

Current behavior

There is no indication within the documentation that --color-dialog and --color-on-dialog need to be defined within your repository in order to set the background and color of the .inner dialog styling https://github.com/reaviz/realayers/blob/master/src/Dialog/Dialog.tsx#L60
https://github.com/reaviz/realayers/blob/master/src/Dialog/Dialog.module.css#L27

Without those defined, there is no .inner background color and is transparent against the .dialog styles

Expected behavior

Documentation specifying any css defined values, including --color-dialog and --color-on-dialog

Minimal reproduction of the problem with instructions

Create a story using the Dialog component without --color-dialog and --color-on-dialog being defined within the app's css files.

<Dialog open={state} onClose={() => setState(false)}>
  <h2>Dialog</h2>
</Dialog>

What is the motivation / use case for changing the behavior?

Faster utilization of the Dialog component

Environment

Libs:
- react version: 17.0.2
- realayers version: 2.4.6


Browser:
- [ ] Chrome (desktop) version XX
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: XX  
- Platform:  

Others: