update readme

Author: chantu

README.md

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 # HandyJSON
 
+**For Xcode 10 (Swift 4.2), switch to branch dev_for_swift_4.2. Notice that only Xcode 10 beta 4 is workable, otherwise you have to download the newest Swift toolchain to run the project.**
+
 HandyJSON is a framework written in Swift which to make converting model objects( **pure classes/structs** ) to and from JSON easy on iOS.
 
 Compared with others, the most significant feature of HandyJSON is that it does not require the objects inherit from NSObject(**not using KVC but reflection**), neither implements a 'mapping' function(**writing value to memory directly to achieve property assignment**).