Add documentation for running the worker locally

[cpliakas]

We need to figure out how to run the worker locally without having to explicitly pass it events, i.e., it should consume SQS messages continuously.

For example, wouldn't it be cool if we could use https://github.com/localstack/localstack in combination with this application to consume the /job endpoint, send a message to a local SQS instance, and then have the worker automatically consume it like it does on Amazon?

[cpliakas]

Leveraging sam local generate-event sqs receive-message could be a "crawl".