AO3-6691 Add Readme for custom RuboCop cops #3908

Job	Run time
rake db:otwseed	1m 15s
rspec spec/controllers	10m 20s
rspec spec/models	5m 38s
rspec --exclude-pattern 'spec/{controllers,models}/*/.rb'	4m 50s
cucumber features/admins	4m 48s
cucumber features/bookmarks	5m 19s
cucumber features/collections	6m 14s
cucumber features/comments_and_kudos	6m 8s
cucumber features/gift_exchanges	6m 44s
cucumber features/importing	2m 32s
cucumber features/other_a	11m 36s
cucumber features/other_b	6m 29s
cucumber features/prompt_memes_a	4m 25s
cucumber features/prompt_memes_b	3m 55s
cucumber features/prompt_memes_c	5m 5s
cucumber features/search	6m 3s
cucumber features/tag_sets	4m 0s
cucumber features/tags_and_wrangling	5m 47s
cucumber features/users	4m 31s
cucumber features/works	15m 59s
	2h 1m 38s

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