Add documentation about rubocop grace period and using new cops
What does this MR do and why?
Follow up of gitlab-org/ruby/gems/gitlab-styles!128 (comment 1110971593).
This MR adds documentation for Rubocop Cop's newly introduced grace period, as well as some instructions on how to use a new Cop.
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Edited by Peter Leitzen