Added README for airflow

Author: abhiramr

Libraries_Tests/airflow/README.md

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+## Notes: 
+
+- Change path for DAGs in ~/airflow/airflow.cfg
+- Run airflow script using python <whatever-path>/<filename>.py
+
+### Commands to test and validate:
+
+- airflow list_dags
+- airflow list_tasks <dag_id>
+- airflow list_tasks <dag_id> --tree
+
+- airflow test <dag_id> <task_id> 2015-06-01
+
+- airflow backfill <dag_id> -s <start_date> -e <end_date>
+
+### To run the local server:
+
+- airflow webserver
+