Add standard header to protect .rubocop.yml

This follows the convention established in [1].

[1]: alphagov/content-publisher#2411

.rubocop.yml

@@ -7,6 +7,16 @@ inherit_mode:
   merge:
     - Exclude
 
+# **************************************************************
+# TRY NOT TO ADD OVERRIDES IN THIS FILE
+#
+# This repo is configured to follow the RuboCop GOV.UK styleguide.
+# Any rules you override here will cause this repo to diverge from
+# the way we write code in all other GOV.UK repos.
+#
+# See https://github.com/alphagov/rubocop-govuk/blob/main/CONTRIBUTING.md
+# **************************************************************
+
 Rails/SaveBang:
   AllowedReceivers:
     - Elasticsearch::Model.client.indices