Add note about legacy platforms to readme

README.md

@@ -30,6 +30,7 @@ class MyViewController: UIViewController {
 
 * [Documentation](http://snapkit.io/docs/)
 * [F.A.Q.](http://snapkit.io/faq/)
+* [Legacy Platforms (< iOS 8.0, < OS X 10.10)](https://github.com/SnapKit/SnapKit/blob/develop/LEGACY_PLATFORMS.md)
 
 ## License