Add contribution note for CHANGELOG.md

Follow up rubocop/rubocop#11563.

CHANGELOG.md

@@ -1,5 +1,12 @@
 # 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-rails/blob/master/changelog/
+  named `{change_type}_{change_description}.md` if the new code introduces user-observable changes.
+  See https://github.com/rubocop/rubocop-rails/blob/master/CONTRIBUTING.md#changelog-entry-format for details.
+-->
+
 ## master (unreleased)
 
 ## 2.17.4 (2022-12-25)

spec/project_spec.rb

@@ -127,8 +127,8 @@
       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

tasks/changelog.rb

@@ -30,7 +30,6 @@ 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