Merge pull request apache#349 from CodingCat/support-worker_dir

add the comments about SPARK_WORKER_DIR

this env variable seems to be forgotten

in many cases we need to set this variable, e.g. in EC2, we have to move the large application log files from the EBS to the ephemeral storage

Author: rxin

conf/spark-env.sh.template

@@ -18,4 +18,4 @@
 # - SPARK_WORKER_MEMORY, to set how much memory to use (e.g. 1000m, 2g)
 # - SPARK_WORKER_PORT / SPARK_WORKER_WEBUI_PORT
 # - SPARK_WORKER_INSTANCES, to set the number of worker processes per node
-
+# - SPARK_WORKER_DIR, to set the working directory of worker processes