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

tools/release/update_node.py

 #!/usr/bin/env python
 # Copyright 2017 the V8 project authors. All rights reserved.
 # 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
 import subprocess
 import sys
 
 TARGET_SUBDIR = os.path.join("deps", "v8")
 
 SUB_REPOSITORIES = [ ["base", "trace_event", "common"],
                      ["testing", "gtest"],
@@ skipping 125 matching lines @@
   UpdateTarget([""], options)
   # Patch .gitignore before updating sub-repositories.
   UpdateGitIgnore(options)
   for repo in SUB_REPOSITORIES:
     UpdateTarget(repo, options)
   if options.commit:
     CreateCommit(options)
 
 if __name__ == "__main__":
   Main(sys.argv[1:])