Clarify README about unique expiration
Clarify README about unique expiration

antek-drzewiecki/sidekiq-unique-jobs,mhenrixon/sidekiq-unique-jobs,fgbreel/sidekiq-unique-jobs,mmustala/sidekiq-unique-jobs,yss44/sidekiq-unique-jobs,mhenrixon/sidekiq-unique-jobs,pik/sidekiq-unique-jobs,CruGlobal/sidekiq-unique-jobs,tsubery/sidekiq-unique-jobs,mhenrixon/sidekiq-unique-jobs,mhenrixon/sidekiq-unique-jobs,Invoca/sidekiq-unique-jobs