Add documentation for nsinit

About how to use and explaination of available options.

Signed-off-by: Sun Jianbo <wonderflow@zju.edu.cn>

nsinit/README.md

@@ -65,3 +65,48 @@ You can identify if a process is running in a container by looking to see if
    
 You may also specify an alternate root directory from where the `container.json`
 file is read and where the `state.json` file will be saved.
+
+### How to use?
+
+Currently nsinit has 9 commands. Type `nsinit -h` to list all of them. 
+And for every alternative command, you can also use `--help` to get more 
+detailed help documents. For example, `nsinit config --help`.
+
+`nsinit` cli application is implemented using [cli.go](https://github.com/codegangsta/cli). 
+Lots of details are handled in cli.go, so the implementation of `nsinit` itself 
+is very clean and clear.
+
+*   **config**	
+It will generate a standard configuration file for a container.  By default, it 
+will generate as the template file in [config.go](https://github.com/docker/libcontainer/blob/master/nsinit/config.go#L192). 
+It will modify the template if you have specified some configuration by options.
+*   **exec**	
+Starts a container and execute a new command inside it. Besides common options, it
+has some special options as below.
+	- `--tty,-t`: allocate a TTY to the container.
+	- `--config`: you can specify a configuration file. By default, it will use 
+	template configuration.
+	- `--id`: specify the ID for a container. By default, the id is "nsinit".
+	- `--user,-u`: set the user, uid, and/or gid for the process. By default the 
+	value is "root".
+	- `--cwd`: set the current working dir.
+	- `--env`: set environment variables for the process.
+*   **init**		
+It's an internal command that is called inside the container's namespaces to 
+initialize the namespace and exec the user's process. It should not be called 
+externally.
+*   **oom**		
+Display oom notifications for a container, you should specify container id.
+*   **pause**	
+Pause the container's processes, you should specify container id. It will use 
+cgroup freeze subsystem to help.
+*   **unpause**		
+Unpause the container's processes. Same with `pause`.
+*   **stats**	
+Display statistics for the container, it will mainly show cgroup and network 
+statistics.
+*   **state**	
+Get the container's current state. You can also read the state from `state.json`
+ in your container_id folder.
+*   **help, h**		
+Shows a list of commands or help for one command.