revise README.md

Author: hechen

README.md

@@ -5,3 +5,25 @@
 Select which file will be opened by which xcode.
 
 ![Main Interface](.assets/Main.png)
+
+
+## Under the hood
+
+1. Register self as the default application that the files with specific extension will be opened with.
+2. When self receive openURL notification, we decide which real application user specified before to open it.
+3. Done
+
+
+## Core
+
+``` Swift
+/*
+Sets the user’s preferred default handler for the specified content type in the specified roles.
+*/
+LSSetDefaultRoleHandlerForContentType(_:_:_:)
+
+/*
+Returns the application bundle identifier of the user’s preferred default handler for the specified content type with the specified role.
+*/
+LSCopyDefaultRoleHandlerForContentType(_:_:)
+```