Extension Readme: Add link to docs and screenshot(s)

[thedanfernandez]

Issue

Einstein for Developers requires setup instructions that, if you discover the extension, you wouldn't know are required to use the extension.

Proposed Fix

Update the extension README.md, after the Overview section with markdown similar to the following:

## Documentation
Please visit the [Einstein for Developers (Beta) documentation](https://developer.salesforce.com/tools/vscode/en/einstein/einstein-overview) for [setup instructions](https://developer.salesforce.com/tools/vscode/en/einstein/einstein-setup)

## Screenshot

![Screenshot of Einstein for Developers generating Apex based on natural language](https://d33wubrfki0l68.cloudfront.net/4b089c34b9e2d170c8ae4dc38c00cf82f47660ef/abad5/images/einstein-sidebar.png)

Example in readme

Documentation

Please visit the Einstein for Developers (Beta) documentation for setup instructions.

Screenshot

[gbockus-sf]

@sbudhirajadoc @AnanyaJha an issue for your consideration