Add documentation for configuring runtime environment with ElasticSearch API credentials

AWS_ACCESS_KEY and AWS_SECRET_KEY need to be added to the local runtime environment if ElasticSearch indexing is to be done. There need to be instructions on how to do this in README.md, like "add export AWS_ACCESS_KEY=KEYID and export AWS_SECRET_KEY=KEY to /etc/crawler/secrets and run source /etc/crawler/secrets prior to running python scan.py --elastic" except more clear and detailed and also covering how to do it in Windows.