add note for systemd-nspawn and rename target to rootfs in examples because cargo has a target directory

README.md

@@ -4,6 +4,12 @@ putting them and their necessary libs into a target rootfs directory
 example:
 
 ```shell
-cargo run -- -v target/ /bin/bash /bin/ls
-sudo chroot target /bin/bash
+cargo run -- -v rootfs/ /bin/bash /bin/ls
+
+# chroot
+sudo chroot rootfs /bin/bash
+
+# systemd nspawn
+wget -O rootfs/sbin/init https://github.com/Yelp/dumb-init/releases/download/v1.2.2/dumb-init_1.2.2_amd64
+systemd-nspawn -D rootfs /sbin/init /bin/bash
 ```