Update doc with swift version and inspiration link

README.md

@@ -1,11 +1,11 @@
 # swift-cpp-package
-Playground project for writing Swift packages which use C++ code
+Playground project for writing Swift packages which use C++ code. Based on instructions at http://ankit.im/swift/2016/05/21/creating-objc-cpp-packages-with-swift-package-manager/#a-package-with-cpp-and-swift
 
 # Prerequesites
-- [Swift 3](https://swift.org/download/)
+- [Swift 4](https://swift.org/download/)
 
 # Build
 `swift build`
 
 # Run
-`.build/debug/swift-exec`
+`.build/debug/swift-exec` (compiled from [Sources/swift-exec/main.swift](https://github.com/itsravenous/swift-cpp-package/blob/master/Sources/swift-exec/main.swift))