Add contribution note for CHANGELOG.md

Follow up rubocop/rubocop#11563.

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-minitest/blob/master/changelog/
+  named `{change_type}_{change_description}.md` if the new code introduces user-observable changes.
+
+  See https://github.com/rubocop/rubocop-minitest/blob/master/CONTRIBUTING.md#changelog-entry-format for details.
+-->
+
 ## master (unreleased)
 
 ## 0.27.0 (2023-01-30)

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

test/project_test.rb

@@ -15,11 +15,11 @@ def test_changelog_has_newline_at_end_of_file
     assert(@changelog.end_with?("\n"))
   end
 
-  def test_changelog_has_either_entries_headers_or_empty_lines
+  def test_changelog_has_either_entries_headers_empty_lines_or_comments
     non_reference_lines = @lines.take_while { |line| !line.start_with?('[@') }
 
     non_reference_lines.each do |line|
-      assert_match(/^(\*|#|$)/, line)
+      assert_match(/^(\*|#|$|<!---|-->|  )/, line)
     end
   end