tmux: add ZSH_TMUX_UNICODE to README

plugins/tmux/README.md

@@ -38,3 +38,4 @@ The plugin also supports the following -
 | `ZSH_TMUX_FIXTERM_WITHOUT_256COLOR` | `$TERM` to use for non 256-color terminals (default: `screen`)                |
 | `ZSH_TMUX_FIXTERM_WITH_256COLOR`    | `$TERM` to use for 256-color terminals (default: `screen-256color`            |
 | `ZSH_TMUX_CONFIG`                   | Set the configuration path (default: `$HOME/.tmux.conf`)                      |
+| `ZSH_TMUX_UNICODE`                  | Set `tmux -u` option to support unicode                                       |