Adds 3.0 release warning to docs.

diff --git a/README.md b/README.md
index 5d82b70..7288a72 100644 (file)
--- a/README.md
+++ b/README.md
@@ -12,6 +12,8 @@ According to [tests](http://goalsmashers.github.io/css-minification-benchmark/)
 
 ## Usage
 
+### This documentation refers to a pre-release version. See [2.2 branch](https://github.com/jakubpawlowicz/clean-css/tree/2.2) for the current release docs.
+
 ### What are the requirements?
 
 ```