Adds pre-release warning to readme file.

diff --git a/README.md b/README.md
index 53703e2..e6fe9da 100644 (file)
--- a/README.md
+++ b/README.md
@@ -13,6 +13,8 @@ According to [tests](http://goalsmashers.github.io/css-minification-benchmark/)
 
 ## Usage
 
+### This documentation refers to a pre-release version of clean-css. See [3.4 branch](https://github.com/jakubpawlowicz/clean-css/tree/3.4) for the current release docs.
+
 ### What are the requirements?
 
 ```