AO3-6691 Add Readme for custom RuboCop cops #3907

Job	Run time
rake db:otwseed	1m 14s
rspec spec/controllers	10m 33s
rspec spec/models	5m 31s
rspec --exclude-pattern 'spec/{controllers,models}/*/.rb'	5m 0s
cucumber features/admins	5m 6s
cucumber features/bookmarks	4m 45s
cucumber features/collections	6m 23s
cucumber features/comments_and_kudos	6m 18s
cucumber features/gift_exchanges	6m 55s
cucumber features/importing	2m 37s
cucumber features/other_a	10m 43s
cucumber features/other_b	6m 24s
cucumber features/prompt_memes_a	4m 6s
cucumber features/prompt_memes_b	3m 34s
cucumber features/prompt_memes_c	4m 52s
cucumber features/search	6m 29s
cucumber features/tag_sets	3m 12s
cucumber features/tags_and_wrangling	5m 53s
cucumber features/users	4m 28s
cucumber features/works	13m 26s
	1h 57m 29s

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