[tools] add script to simplify backporting patch to Node.js

tools/release/update_node.py

Index: tools/release/update_node.py
diff --git a/tools/release/update_node.py b/tools/release/update_node.py
index e05f71234da4e3905a9d6fac989b8c14ec437ca4..6232fe05bd13134707cc1b4b7f11269ea47cc5d1 100755
--- a/tools/release/update_node.py
+++ b/tools/release/update_node.py
@@ -3,6 +3,26 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+"""
+Use this script to update V8 in a Node.js checkout.
+
+Requirements:
+  - Node.js checkout in which V8 should be updated.
+  - V8 checkout at the commit to which Node.js should be updated.
+
+Usage:
+  $ update_node.py <path_to_v8> <path_to_node>
+
+  This will synchronize the content of <path_to_node>/deps/v8 with <path_to_v8>,
+  and a few V8 dependencies require in Node.js. It will also update .gitignore
+  appropriately.
+
+Optional flags:
+  --gclient     Run `gclient sync` on the V8 checkout before updating.
+  --commit      Create commit with the updated V8 in the Node.js checkout.
+  --with-patch  Also include currently staged files in the V8 checkout.
+"""
+
 import argparse
 import os
 import shutil