[SPARK-2457] Inconsistent description in README about build option

Now, we should use -Pyarn instead of SPARK_YARN when building but README says as follows.

    For Apache Hadoop 2.2.X, 2.1.X, 2.0.X, 0.23.x, Cloudera CDH MRv2, and other Hadoop versions
    with YARN, also set `SPARK_YARN=true`:

      # Apache Hadoop 2.0.5-alpha
      $ sbt/sbt -Dhadoop.version=2.0.5-alpha -Pyarn assembly

      # Cloudera CDH 4.2.0 with MapReduce v2
      $ sbt/sbt -Dhadoop.version=2.0.0-cdh4.2.0 -Pyarn assembly

      # Apache Hadoop 2.2.X and newer
      $ sbt/sbt -Dhadoop.version=2.2.0 -Pyarn assembly

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #1382 from sarutak/SPARK-2457 and squashes the following commits:

e7b2d64 [Kousuke Saruta] Replaced "SPARK_YARN=true" with "-Pyarn" in README

README.md

@@ -81,7 +81,7 @@ versions without YARN, use:
     $ sbt/sbt -Dhadoop.version=2.0.0-mr1-cdh4.2.0 assembly
 
 For Apache Hadoop 2.2.X, 2.1.X, 2.0.X, 0.23.x, Cloudera CDH MRv2, and other Hadoop versions
-with YARN, also set `SPARK_YARN=true`:
+with YARN, also set `-Pyarn`:
 
     # Apache Hadoop 2.0.5-alpha
     $ sbt/sbt -Dhadoop.version=2.0.5-alpha -Pyarn assembly