Document all_queues.yml in sidekiq_style_guide.md

doc/development/sidekiq_style_guide.md

@@ -17,6 +17,9 @@ would be `process_something`. If you're not sure what queue a worker uses,
 you can find it using `SomeWorker.queue`. There is almost never a reason to
 manually override the queue name using `sidekiq_options queue: :some_queue`.
 
+You must always add any new queues to `gitlab/app/workers/all_queues.yml`
+otherwise your worker will not run.
+
 ## Queue Namespaces
 
 While different workers cannot share a queue, they can share a queue namespace.