Update the README(s)

[nathan-schmidt-viget]

Need to add in info about:

• Update the readme files to add in link to the WP Global Settings & Styles and Theme.json reference - Theme
• Link from the main README to the theme README - Main
• Adding custom button icons - Theme
• Info about setting the button style in WP and in TW - Theme
• Limiting colors on a per-block basis - Theme

[andrewmosby]

I think this issue could include the link from the main README to the theme README.