Document asynchronous mode in README.
Test the functionality of the DocumentQueue.
Make the indexing worker retry fewer times.

The default number of retries is 25, with exponential backoff, which
will cause a request to be retried for about three weeks after it's
initially submitted. By this time, there's a good chance the document
will be out of date anyway. Five retries over about 7 minutes seems like
a far more sensible default.

See <https://github.com/mperham/sidekiq/wiki/Error-Handling>.
Test the functionality of the indexing worker.