Add contribution note for CHANGELOG.md

Occasionally I review direct edit to the CHANGELOG.md:

- #11559 (comment)
- #11406 (comment)
- #11395 (comment)

It might be communicated a little more efficiently if listed in the CHANGELOG.md.
And the triple dash for commenting out is used intentionally.
https://stackoverflow.com/questions/4823468/comments-in-markdown/4829998#4829998

It affects when markdown is viewed on GitHub. This is unnecessary note for
user viewing the CHANGELOG.md on GitHub.

Also add changelog/.gitkeep to prevent dead link after release.

CHANGELOG.md

@@ -1,5 +1,14 @@
 # Change log
 
+<!---
+  Do NOT edit this CHANGELOG.md file by hand directly, as it is automatically updated.
+
+  Please add an entry file to the https://github.com/rubocop/rubocop/blob/master/changelog/
+  named `{change_type}_{change_description}.md` if the new code introduces user-observable changes.
+
+  See https://github.com/rubocop/rubocop/blob/master/CONTRIBUTING.md#changelog-entry-format for details.
+-->
+
 ## master (unreleased)
 
 ## 1.45.1 (2023-02-08)

spec/project_spec.rb

@@ -156,8 +156,8 @@ def expected
       expect(changelog.end_with?("\n")).to be true
     end
 
-    it 'has either entries, headers, or empty lines' do
-      expect(non_reference_lines).to all(match(/^(\*|#|$)/))
+    it 'has either entries, headers, empty lines, or comments' do
+      expect(non_reference_lines).to all(match(/^(\*|#|$|<!---|-->|  )/))
     end
 
     describe 'entry' do

spec/tasks/changelog_spec.rb

@@ -8,6 +8,15 @@
     described_class.new(content: <<~CHANGELOG, entries: list)
       # Change log
 
+      <!---
+        Do NOT edit this CHANGELOG.md file by hand directly, as it is automatically updated.
+
+        Please add an entry file to the https://github.com/rubocop/rubocop/blob/master/changelog/
+        named `{change_type}_{change_description}.md` if the new code introduces user-observable changes.
+
+        See https://github.com/rubocop/rubocop/blob/master/CONTRIBUTING.md#changelog-entry-format for details.
+      -->
+
       ## master (unreleased)
 
       ### New features

tasks/changelog.rb

@@ -25,7 +25,6 @@ def initialize(type:, body: last_commit_title, ref_type: nil, ref_id: nil, user:
     end
 
     def write
-      FileUtils.mkdir_p(ENTRIES_PATH)
       File.write(path, content)
       path
     end