Add Swift version notice on the top

README.md

@@ -7,7 +7,9 @@
 
 # CryptoSwift
 
-Crypto related functions and helpers for [Swift](https://swift.org) implemented in Swift. ([#PureSwift](https://twitter.com/hashtag/pureswift)) 
+Crypto related functions and helpers for [Swift](https://swift.org) implemented in Swift. ([#PureSwift](https://twitter.com/hashtag/pureswift))
+
+**Notice**: The master branch follows the latest currently released version of Swift. If you need an version for older version of Swift, you can specify it's version in your Podfile or use the code on the branch for that version. Older branches are unsupported. Check [versions](#swift-versions-support) for details.
 
 # Table of Contents
 - [Requirements](#requirements)