SPARK-5390 [DOCS] Encourage users to post on Stack Overflow in Community Docs

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

docs/index.md

@@ -115,6 +115,8 @@ options for deployment:
 
 * [Spark Homepage](http://spark.apache.org)
 * [Spark Wiki](https://cwiki.apache.org/confluence/display/SPARK)
+* [Spark Community](http://spark.apache.org/community.html) resources, including local meetups
+* [StackOverflow tag `apache-spark`](http://stackoverflow.com/questions/tagged/apache-spark)
 * [Mailing Lists](http://spark.apache.org/mailing-lists.html): ask questions about Spark here
 * [AMP Camps](http://ampcamp.berkeley.edu/): a series of training camps at UC Berkeley that featured talks and
   exercises about Spark, Spark Streaming, Mesos, and more. [Videos](http://ampcamp.berkeley.edu/3/),
@@ -123,11 +125,3 @@ options for deployment:
 * [Code Examples](http://spark.apache.org/examples.html): more are also available in the `examples` subfolder of Spark ([Scala]({{site.SPARK_GITHUB_URL}}/tree/master/examples/src/main/scala/org/apache/spark/examples),
  [Java]({{site.SPARK_GITHUB_URL}}/tree/master/examples/src/main/java/org/apache/spark/examples),
  [Python]({{site.SPARK_GITHUB_URL}}/tree/master/examples/src/main/python))
-
-# Community
-
-To get help using Spark or keep up with Spark development, sign up for the [user mailing list](http://spark.apache.org/mailing-lists.html).
-
-If you're in the San Francisco Bay Area, there's a regular [Spark meetup](http://www.meetup.com/spark-users/) every few weeks. Come by to meet the developers and other users.
-
-Finally, if you'd like to contribute code to Spark, read [how to contribute](contributing-to-spark.html).