Fix README.md.

README.md uses syntax highlighting and fenced code sections, both features from Github-flavored Markdown.

It works as intended when viewing the project on Github, but rendering is broken when included in an Xcode docset[1]. This is because Appledoc uses libDiscount rather than a more compatible parser such as libSundown.

Updated link for Requirements/What's New in OS X to a goo.gl URL for the same reason. Also corrected OS name to match the usage at the target URL.

[1]http://imgur.com/a/st7Wd