Add placeholder .mailmap file

People change their names and email addresses for many reasons. Adding
a `.mailmap` ensures that e.g. `git log` and `git blame` respect
people’s choices.

Bug: chromium:1213438
Change-Id: I03fcacff90d996d423283c345ddfc4ed9fccf98e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2919671Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74799}

.mailmap

+# Copyright 2021 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.
+
+# People change their names or email addresses for many reasons. This
+# list makes sure people’s preferences are respected when using Git
+# commands such as `git log` or `git blame`.
+#
+# See https://git-scm.com/docs/gitmailmap for more information.
+#
+# Please keep the list sorted.