Add information about Sidekiq to README

In response to [this comment](https://github.com/jrgifford/delayed_paperclip/pull/15#issuecomment-21484145) by @jrgifford I went ahead and added a couple of lines to the README about how to use `delayed_paperclip` with Sidekiq.

What do you think?

Edit: And maybe you could change the repo description to include Sidekiq too so that other people can easily find this?