readme: add intruction to install nydus-snapshotter

Signed-off-by: Changwei Ge <[email protected]>

README.md

@@ -49,8 +49,13 @@ Please follow instructions to [configure nydus](./docs/configure_nydus.md) confi
 
 Nydus-snapshotter is implemented as a [proxy plugin](https://github.com/containerd/containerd/blob/04985039cede6aafbb7dfb3206c9c4d04e2f924d/PLUGINS.md#proxy-plugins) (`containerd-nydus-grpc`) for containerd.
 
-A example of starting nydus-snapshotter:
+Assume your server systemd based, install nydus-snapshotter:
+```bash
+make install
+systemctl restart containerd
+```
 
+Or you can start nydus-snapshotter manually
 ```bash
 # `nydusd-path` is the path to nydusd binary
 # `address` is the domain socket that you configured in containerd configuration file