Merge pull request #488 from ZJU-SEL/add_nsinit_readme

add readme for nsinit about how to build nsinit

nsinit/README.md

@@ -0,0 +1,57 @@
+## nsinit
+
+`nsinit` is a cli application which demonstrates the use of libcontainer.  
+It is able to spawn new containers or join existing containers.  
+
+### How to build?
+
+First to add the `libcontainer/vendor` into your GOPATH. It's because something related with this [issue](https://github.com/docker/libcontainer/issues/210).
+
+```
+export GOPATH=$GOPATH:/your/path/to/libcontainer/vendor
+```
+
+Then get into the nsinit folder and get the imported file. Use `make` command to make the nsinit binary.
+
+```
+cd libcontainer/nsinit
+go get
+make
+```
+
+We have finished compiling the nsinit package, but a root filesystem must be provided for use along with a container configuration file.
+
+Choose a proper place to run your container. For example we use `/busybox`.
+
+```
+mkdir /busybox 
+curl -sSL 'https://github.com/jpetazzo/docker-busybox/raw/buildroot-2014.11/rootfs.tar' | tar -xC /busybox
+```
+
+Then you may need to write a configure file named `container.json` in the `/busybox` folder.
+Environment, networking, and different capabilities for the container are specified in this file.
+The configuration is used for each process executed inside the container
+See the `sample_configs` folder for examples of what the container configuration should look like.
+
+```
+cp libcontainer/sample_configs/minimal.json /busybox/container.json
+cd /busybox
+```
+
+Now the nsinit is ready to work.
+To execute `/bin/bash` in the current directory as a container just run the following **as root**:
+```bash
+nsinit exec --tty /bin/bash
+```
+
+If you wish to spawn another process inside the container while your 
+current bash session is running, run the same command again to 
+get another bash shell (or change the command).  If the original 
+process (PID 1) dies, all other processes spawned inside the container 
+will be killed and the namespace will be removed. 
+
+You can identify if a process is running in a container by 
+looking to see if `state.json` is in the root of the directory.
+   
+You may also specify an alternate root place where 
+the `container.json` file is read and where the `state.json` file will be saved.