Update README to tell specifically about Esprima-FB

* Added info on Esprima-FB with corresponding links to spec.
* Updated reference links to AST and tests pages.
* Added short explanation on versioning based on @zpao's comment: https://github.com/facebook/esprima/issues/35#issuecomment-52277629

Closes #40.
Though it makes sense to note that it's upstream is not Esprima's master but Harmony branch (which powers ES6 features).