Update README to include tmux 2.1 support

Author: tony

README.rst

@@ -45,7 +45,7 @@ Project details
 ---------------
 
 ==============  ==========================================================
-tmux support    1.8, 1.9a, 2.0
+tmux support    1.8, 1.9a, 2.0, 2.1
 python support  2.6, 2.7, >= 3.3
 config support  yaml, json, python dict
 Source          https://github.com/tony/tmuxp