Rewrite the README to describe FRP and RAC in detail

I thought I'd take a stab a writing a more detailed README for RAC. This reflects my understanding of RAC and FRP, which may not be entirely correct. So please let me know if there are any inaccuracies.

I wanted RAC to have a README that:

 * Explained FRP
 * Tried to explain why you would use RAC
 * Didn't link to a blog post for more information
 * Could evolve with the project

The content borrows generously from @joshaber's slides and gists.

There's still room for improvement, either by adding more detail to the README or by adding separate docs. I'd like to see information about streams, sequences, etc.

[Rendered Markdown](https://github.com/mdiep/ReactiveCocoa/blob/detailed-readme/README.md)