SPARK-1911 [DOCS] Warn users if their assembly jars are not built with Java 6

Add warning about building with Java 7+ and running the JAR on early Java 6.

CC andrewor14

Author: Sean Owen <sowen@cloudera.com>

Closes #4874 from srowen/SPARK-1911 and squashes the following commits:

79fa2f6 [Sean Owen] Add warning about building with Java 7+ and running the JAR on early Java 6.