document synchronized panes in tmux

tmux.md

@@ -18,6 +18,7 @@ Simply invoke `tmux` to start a new tmux session. You will notice a new (customi
 | "           | splits the current pane into two horizontal panes                                     |
 | cursor keys | switches between panes                                                                |
 | !           | break out current pane to new window                                                  |
+| :           | enter tmux command                                                                    |
 
 ## Walkthrough: detaching and attaching
 
@@ -30,3 +31,22 @@ echo "I am a plain terminal"
 tmux ls
 tmux a -t mysession
 ```
+
+## Enter tmux commands
+
+tmux has a command interface as well. You can enter it by pressing prefix followed by `:`. You will see a input line at the bottom of your tmux session.
+
+###  Walkthrough: synchronize the input to multiple panes
+
+```bash
+tmux
+# split pane vertically
+Ctrl+b, %
+# syncronize-panes
+Ctrl+b, :
+setw synchronize-panes
+echo hello
+# de-syncronize-panes
+Ctrl+b, :
+setw synchronize-panes
+```