Updating README.md

Updating the global Rails README following work by @strzalek et al, which extracted Action View to a separate gem. Also, some other improvements to the README to reflect the current state of the framework.

Summary of changes:

* Reordered the paragraphs from View-Model-Controller to Model-Controller-View, based on how (IMHO) most people think about the structure and hierarchy of the framework and their code in it.
* Added references to ActionView's README in the _View_ paragraph.
* Reworded the _Controller_ paragraph to briefly mentioned routing as handled by Action Dispatch, and that both Action Dispatch and Action Controller work together to form the request-handling Action Pack.
* Removed paragraph saying Action Pack is coupled with Action View, as this is no longer the case.
* Added brief references to Action Mailer and Active Support, since they are both major Rails components. Briefly mentioned in the _View_ paragraph that Action View is often used to generate the body of the email, in addition to the other typical of using to build a response body.