Update DEVELOPER-ADVANCED.md with Sidekiq info

I was confused as to why activation emails weren't showing up in Mailcatcher, and finally took a look at the non-advanced developer guide to find the mention of Sidekiq. I thought it would be appropriate to mention it here as well.
Merge pull request #2487 from toolness/patch-2

Update DEVELOPER-ADVANCED.md with Sidekiq info