Point "Community" to main Spark Community page; mention SO tag apache-spark.

Separately, the Apache site can be updated to mention, under Mailing Lists:
"StackOverflow also has an apache-spark tag for Spark Q&A." or similar.

Author: Sean Owen <sowen@cloudera.com>

Closes #4843 from srowen/SPARK-5390 and squashes the following commits:

3508ac6 [Sean Owen] Point "Community" to main Spark Community page; mention SO tag apache-spark