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.
Merge pull request #50 from mrnugget/sidekiq_to_readme

Add information about Sidekiq to README