Add a note about tmux 2.5 breaking changes

Author: bruno-

README.md

@@ -2,6 +2,9 @@
 
 [![Build Status](https://travis-ci.org/tmux-plugins/tmux-copycat.png?branch=master)](https://travis-ci.org/tmux-plugins/tmux-copycat)
 
+**NOTE**: the changes in tmux 2.4 broke this plugin. I'm working on a fix. For
+now the plugin works on tmux 2.3.
+
 This plugin enables:
 
 - regex searches