add tmux 2.3 to README.rst for supported ver

Author: tony

README.rst

@@ -178,7 +178,7 @@ Project details
 ---------------
 
 ==============  ==========================================================
-tmux support    1.8, 1.9a, 2.0, 2.1, 2.2
+tmux support    1.8, 1.9a, 2.0, 2.1, 2.2, 2.3
 python support  2.6, 2.7, >= 3.3
 Source          https://github.com/tony/libtmux
 Docs            http://libtmux.git-pull.com