Add a "Why not RxJS" section to the README

[appsforartists]

• Consistency with other platforms that implement Material Motion
• Full control of the stack:
  • Can ignore concepts we don't need, like error and complete
  • Fewer concepts -> smaller learning curve for users
  • Minimize bundle size by only implementing what we use
• Fully declarative - promotes toolability

Now that RxJS allows composition with let, an RxJS port might be worth exploring.