added README.md

sql/README.md

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+SQL Scripts
+===========
+
+`aws_athena_cloudtrail_ddl.sql` - [AWS Athena](https://aws.amazon.com/athena/) DDL to setup up integration to [CloudTrail](https://aws.amazon.com/cloudtrail/) logs from Athena
+`bigquery_billing_*.sql` - various [Google BigQuery](https://cloud.google.com/bigquery) billing queries