Add links for adding docsets to Xcode via docs-for-xcode://.

Xcode 5 no longer has UI for adding feed URLs, but still supports
3rdparty docsets; they just have to be installed manually. Add links for
installing docsets using a dedicated app that does this, Docs for Xcode,
and its custom protocol.

The end result is that adding a docset into Xcode is as convenient as
adding into Dash.
Merge pull request #59 from vslavik/docs-for-xcode

Add support for installing docsets into Xcode 5.