disable multi-threading in spark-env.sh

Author: Peng Meng

conf/spark-env.sh.template

@@ -61,3 +61,7 @@
 # - SPARK_IDENT_STRING  A string representing this instance of spark. (Default: $USER)
 # - SPARK_NICENESS      The scheduling priority for daemons. (Default: 0)
 # - SPARK_NO_DAEMONIZE  Run the proposed command in the foreground. It will not output a PID file.
+# Options for native BLAS, like Intel MKL, OpenBLAS, and so on.
+# You shoud enable these options if use native BLAS (SPARK-21305).
+# - MKL_NUM_THREADS=1        Disable multi-threading of Intel MKL
+# - OPENBLAS_NUM_THREADS=1   Disable multi-threading of OpenBLAS