rails/rails — pull request #11521 by mking (2013-07-20 19:42:03+00:00)
Title: 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 each 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).