tmux: add README (#7293)

plugins/tmux/README.md

+# tmux
+
+This plugin provides aliases for [tmux](http://tmux.github.io/), the terminal multiplexer.
+To use it add `tmux` to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... tmux)
+```
+
+The plugin also supports the following - 
+- determines if tmux is installed or not, if not, prompts user to install tmux
+- determines if the terminal supports the 256 colors or not, sets the appropriate configuration variable
+- sets the correct local config file to use
+
+## Aliases
+
+| Alias  | Command                | Description                                               |
+| ------ | -----------------------|---------------------------------------------------------- |
+| `ta`   | tmux attach -t         | Attach new tmux session to already running named session  |
+| `tad`  | tmux attach -d -t      | Detach named tmux session                                 |
+| `ts`   | tmux new-session -s    | Create a new named tmux session                           |
+| `tl`   | tmux list-sessions     | Displays a list of running tmux sessions                  |
+| `tksv` | tmux kill-server       | Terminate all running tmux sessions                       |
+| `tkss` | tmux kill-session -t   | Terminate named running tmux session                      |
+| `tmux` | `_zsh_tmux_plugin_run` | Start a new tmux session                                  |
+
+
+## Configuration Variables
+
+| Variable                            | Description                                                                   |
+|-------------------------------------|-------------------------------------------------------------------------------|
+| `ZSH_TMUX_AUTOSTART`                | Automatically starts tmux (default: `false`)                                  |
+| `ZSH_TMUX_AUTOSTART_ONCE`           | Autostart only if tmux hasn't been started previously (default: `true`)       |
+| `ZSH_TMUX_AUTOCONNECT`              | Automatically connect to a previous session if it exits (default: `true`)     |
+| `ZSH_TMUX_AUTOQUIT`                 | Automatically closes terminal once tmux exits (default: `ZSH_TMUX_AUTOSTART`) |
+| `ZSH_TMUX_FIXTERM`                  | Sets `$TERM` to 256-color term or not based on current terminal support       |
+| `ZSH_TMUX_ITERM2`                   | Sets the `-CC` option for iTerm2 tmux integration (default: `false`)          |
+| `ZSH_TMUX_FIXTERM_WITHOUT_256COLOR` | `$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`            |