Add .asf.yaml to control Github settings

We added .asf.yaml file to Apache Airflow project (PR) and I think it would be good to have the Spark's website at the top of Github repo.

Also, this would allow Spark's PMC Members and committer to control common Github project settings by themselves without having to ask Apache INFRA team.

More info: https://cwiki.apache.org/confluence/display/INFRA/.asf.yaml+features+for+git+repositories

We (Airflow PMC and Committers) used this file to enable Github issues and disable PR Merge button etc