Rails 4.0 release notes. Explain major features added and removed.

This resolves the TODO in the Rails 4.0 release notes. It provides an explanation for each major feature listed in the "Major Features" diagram, and provides links to commits.

This should be helpful to newcomers to Rails. For instance, while reading "Major Features" diagram, I had to look up the abbreviations (example: AP, AV, AC).

Possible issues:

* I linked "New deprecation policy" to a video because I could not find a definitive source for this.
* I did not provide a link or explanation for Google security changes, because I was not sure what this referred to.