Update changelog for 0.6.0

Author: tony

CHANGES

@@ -5,6 +5,8 @@ Changelog
 Here you can find the recent changes to libtmux
 
 - :release:`0.6.0 <2016-09-16>`
+- :support:`-` Raise exception for invalid session names. tmux does not
+  allow names that are empty, contain periods or colons.
 - :bug:`-` Remove unused ``target_sesssion`` param in
   ``Server.attach_session`` and ``Server.switch_client``.