[SPARK-11056] Improve documentation of SBT build.

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 apache#9068 from kayousterhout/SPARK-11056.

docs/building-spark.md

@@ -216,6 +216,11 @@ can be set to control the SBT build. For example:
 
     build/sbt -Pyarn -Phadoop-2.3 assembly
 
+To avoid the overhead of launching sbt each time you need to re-compile, you can launch sbt
+in interactive mode by running `build/sbt`, and then run all build commands at the command
+prompt. For more recommendations on reducing build time, refer to the
+[wiki page](https://cwiki.apache.org/confluence/display/SPARK/Useful+Developer+Tools#UsefulDeveloperTools-ReducingBuildTimes).
+
 # Testing with SBT
 
 Some of the tests require Spark to be packaged first, so always run `build/sbt assembly` the first time.  The following is an example of a correct (build, test) sequence: