add drivers-build README.md

Author: bennsimon

docker/drivers-build/README.md

@@ -0,0 +1,6 @@
+# Overview
+
+The docker/drivers-build directory contains a sample Dockerfile for packaging postgresql database JDBC driver to be used by spark-thriftserver as an external storage. See demonstration [docker compose file](../compose-controller-spark-sql-external-storage.yaml). 
+
+
+ > This folder should be ignored during continuous integration tests.