Update README with Slim 2 notice

Author: craig-davis

README.md

@@ -1,6 +1,10 @@
 Slim Unit Testing Example [![Build Status](https://travis-ci.org/there4/slim-unit-testing-example.png?branch=master)](https://travis-ci.org/there4/slim-unit-testing-example) [![Code Climate](https://codeclimate.com/github/there4/slim-unit-testing-example/badges/gpa.svg)](https://codeclimate.com/github/there4/slim-unit-testing-example)
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-> Integration and unit testing a Slim PHP application
+> Integration and unit testing a Slim PHP application (Slim V2.x)
+
+## Slim V2.x
+The current stable master of this project is for Slim V2.x. Work is in progress
+to bring this to the new Slim 3.
 
 This is a sample application to show an approach to integration and unit testing
 a [Slim][slim] application. To skip to the heart of this, go check out the