readme 추가

README.md

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+# init-wrapper
+It performs the same operation as the shell at the bottom.
+
+```
+#!/bin/sh
+set -eu
+mount --make-private /
+mount -t tmpfs -o lazytime,relatime,nodev,nosuid parentfs /run
+mkdir -p /run/overlay
+mkdir -p /run/overlay/lower
+mkdir -p /run/overlay/upper
+mkdir -p /run/overlay/work
+mkdir -p /run/overlay/merged
+mount -o bind / /run/overlay/lower
+mount -t overlay -o rw,relatime,lowerdir=/run/overlay/lower,upperdir=/run/overlay/upper,workdir=/run/overlay/work,redirect_dir=on,uuid=on,metacopy=on,volatile rootfs /run/overlay/merged
+mkdir -p /run/overlay/merged/oldroot
+pivot_root /run/overlay/merged /run/overlay/merged/oldroot
+cd /
+mount  --move /oldroot/run /run
+umount -l /oldroot
+rmdir /oldroot
+rmdir /run/overlay/merged
+exec /sbin/init
+```