add the comments about SPARK_WORKER_DIR

this env variable seems to be forgotten …

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