Add information about Sidekiq to README
Add information about Sidekiq to README

This adds a couple of lines to the README about Sidekiq, explaining that
it's possible to use `delayed_paperclip` with Sidekiq. It also explains
which queue Sidekiq has to listen to in order for the setup to work.

leighhalliday/delayed_paperclip,jrgifford/delayed_paperclip,ombulabs/delayed_paperclip,hewrin/delayed_paperclip,cgunther/delayed_paperclip,owahab/delayed_paperclip,matt0903/delayed_paperclip