add "tmux -2" note

Author: seebi

README.md

@@ -9,6 +9,9 @@ A tmux solarized theme using Ethan Schoonover’s [Solarized color scheme](http:
 ## Installation
 This config snippet for the terminal multiplexer tmux should be added to your `~/.tmux.conf` configuration file.
 
+In most cases, you have to force tmux to assume the terminal supports 256 colours.
+For this, start tmux as `tmux -2`.
+
 ## Screenshot
 Here is a screenshot of a tmux session captured from a gnome-terminal using the [dz-version of the awesome Inconsolata font](http://nodnod.net/2009/feb/12/adding-straight-single-and-double-quotes-inconsola/).