update readme

[erikdstock]

Followup from #2
Cover:

• setup (need to verify this)
• emotion
• rx prop + jsx pragma
• styled-system theme interface plus our select additional keys
• defining components and variants using withStyleProps
• defining components and variants without it (eg Input)
• motivation:
  • improved type support
  • opinionated theme interface with more stable top-level keys
  • option to define core styles in one place (theme.componentStyles, maybe htmlStyles in the future for things like mdx support)
  • keep styles in rx prop:
    • easier maintenance (ITO types and rather than adding and keeping track of dozens of them)
    • more familiar to the most vanilla react component styles prop (+ styled-system niceties)