Update README.

Author: luosheng

README.md

@@ -1,6 +1,6 @@
 # OpenSim [![Travis](https://img.shields.io/travis/luosheng/OpenSim.svg)]()
 
-OpenSim is an open source alternative to [SimPholders](https://simpholders.com), written in Swift. It's working for Xcode 7.
+OpenSim is an open source alternative to [SimPholders](https://simpholders.com), written in Swift 3. If you are looking for sources with Swift 2, please check out the `swift-2.2` branch.
 
 You can visit the latest [release](https://github.com/luosheng/OpenSim/releases) to grab a compiled version. (Warning: It's not code-signed.)