Documentation: add jazzy

# Jazzy

jazzy is a command-line utility that generates documentation for your Swift or
Objective-C projects.

Only Swift projects are currently supported, but Objective-C support is coming
soon!

Instead of parsing your source files, jazzy hooks into [Clang][clang] and
[SourceKit][sourcekit] to use the [AST][ast] representation of your code and
its comments for more accurate results.

jazzy’s output matches the look & feel of Apple’s official reference
documentation, post WWDC 2014.

[clang]: http://clang.llvm.org "Clang"
[sourcekit]: http://www.jpsim.com/uncovering-sourcekit "Uncovering SourceKit"
[ast]: http://clang.llvm.org/docs/IntroductionToTheClangAST.html "Introduction To The Clang AST"