Add instructions for adding entries to about_flags.cc

Change-Id: I4ee9d935438193d79bd5e6bdd25a6ef186d8a057
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3132327
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#916812}

chrome/browser/about_flags.cc

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// Instructions for adding new entries to this file:
+// https://chromium.googlesource.com/chromium/src/+/main/docs/how_to_add_your_feature_flag.md#step-2_adding-the-feature-flag-to-the-chrome_flags-ui
+
 #include "chrome/browser/about_flags.h"
 
 #include <iterator>