Metrics - Add actions/README.md

tools/metrics/actions/actions.xml

 <!--
 Copyright 2014 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 comprehensive list of Chrome user actions along with the owner
-and description for each user action. A user action should be tied to an actual
-action taken by a user. It should be logged immediately after a user takes the
-action, and should only be logged once per actual event. If an action is not
-user triggered, specify it using not_user_triggered="true" as an attribute to
-the <action> tag.
+and description for each user action.
+
+For best practices on writing user action descriptions, see
+https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/actions/README.md
+For details on how to modify this file to add your description, keep reading.
+
+If an action is not user triggered, specify it using not_user_triggered="true"
+as an attribute to the <action> tag.
 
 Please run extract_actions.py to add new actions and pretty-print this file.
 
 If a user action is not being used any more, put an <obsolete> tag under
 the <action> tag with necessary explanation. Don't delete things in this file.
 
 TODO(yiyaoliu):
 Currently, only file-level comments are supported. By running
 extract_actions.py, comments at other places will be deleted. Ideally, comments
 should be able to be added at any place in this file.
@@ skipping 15842 matching lines @@
 <action name="webapps.AddShortcut.Manifest">
   <owner>mlamouri@chromium.org</owner>
   <description>
     Shortcut added to the homescreen with a valid Manifest. This action will be
     recorded in addition to AppShortcut, AppShortcutApple and Bookmark actions
     to keep a trend of whether the webapp also add meta tags.
   </description>
 </action>
 
 </actions>