Clarify README about unique expiration

I wanted to make sure I understood how long the uniqueness of a job lasts and, if my observation is correct, update the README accordingly. 

I'm confident that the job will be unique for the shorter of the seconds configured or seconds for the job to complete. See: https://github.com/mhenrixon/sidekiq-unique-jobs/blob/master/lib/sidekiq-unique-jobs/middleware/client/unique_jobs.rb#L47

I'm less confident about my observation with Sidekiq versions before 3.0, but I'm pretty sure it's the reason why my long running jobs do not remain unique. See: https://github.com/mperham/sidekiq/issues/1508#issuecomment-37523281 and https://github.com/mperham/sidekiq/blob/20b7a4fe47585ce9c43981305d146bd087540bbf/lib/sidekiq/web_helpers.rb#L50-L83

Thanks!