EasyAuth README: A Novel

Closes #16 

@bcardarella First attempt at documenting EasyAuth. Did my best to deduce what is happening where, but I'm sure I may have misunderstood a thing here or there.

Once this gets merged in, I'll build out provider-specific READMEs with appropriate config information.
"Provider" is usually a term for OAuth apps. I think it is better to say:

"An Identity represents a strategy for authentication. Strategies are flexible enough to support Password, OAuth, OAuth2, or any custom strategy you require."