Updated README explaining Carthage branch situation.
Updated README explaining Carthage branch situation.

SheffieldKevin/SwiftGraphics,slightair/SwiftGraphics,marinehero/SwiftGraphics,sdesimone/SwiftGraphics,Pretz/SwiftGraphics,slightair/SwiftGraphics,SheffieldKevin/SwiftGraphics,Pretz/SwiftGraphics,marinehero/SwiftGraphics,Pretz/SwiftGraphics,hejunbinlan/SwiftGraphics,slightair/SwiftGraphics,SheffieldKevin/SwiftGraphics,slightair/SwiftGraphics,hejunbinlan/SwiftGraphics,sdesimone/SwiftGraphics,marinehero/SwiftGraphics,marinehero/SwiftGraphics,hejunbinlan/SwiftGraphics,sdesimone/SwiftGraphics,Pretz/SwiftGraphics,hejunbinlan/SwiftGraphics,sdesimone/SwiftGraphics