This commit improves the documentation around building Spark to
(1) recommend using SBT interactive mode to avoid the overhead of
launching SBT and (2) refer to the wiki page that documents using
SPARK_PREPEND_CLASSES to avoid creating the assembly jar for each
compile.

cc srowen

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #9068 from kayousterhout/SPARK-11056.