Add standard header to protect .rubocop.yml

https://trello.com/c/Cr2TnMtI/193-make-it-easier-to-agree-with-rubocop-govuk

This follows the convention established in [1].

[1]: alphagov/content-publisher#2411

.rubocop.yml

@@ -6,3 +6,13 @@ inherit_gem:
 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
+# **************************************************************