TestWorker docs - see #413

See #413
Do you think it's a good idea to give a more concrete testing example? Something like:
```python
from redis import Redis
from rq import SimpleWorker , Queue

queue = Queue(connection=Redis())
queue.enqueue(my_long_running_job)
worker = SimpleWorker([queue])
worker.work(burst=True)  # Runs enqueued job
# Check for result...
```