Add a nacl module_args file and a README.

Also a code review settings file.

Review URL:
diff --git a/README.txt b/README.txt
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+++ b/README.txt
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+This repository contains a build directory for standalone native client
+checkouts. In a standalone Native Client checkout, the root directory is not
+under source control (everything is DEPSed in underneath it) so all toplevel
+directories must be separate repositories. This is needed for some GN
+configuration so that it is in an absolute location relative to the source
diff --git a/codereview.settings b/codereview.settings
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+++ b/codereview.settings
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+# This file is used by git-cl to get repository specific information.
diff --git a/module_args/nacl.gni b/module_args/nacl.gni
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+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+# This file is a repository-specific way to specify Native Client build
+# configuration. In a standalone Native Client checkout, this file exists
+# as "//build/module_args/nacl.gni". In a Chrome checkout, a different file
+# exists at that path with Chrome-specific values of the variables.
+# The directory of the GN build configuration. This varies between a standalone
+# Native Client checkout (where it's in "//native_client/build") and Chrome
+# (where it's "//build").
+nacl_shared_build_dir = "//native_client/build"