ART: Adds a help message for parallel compilation

This patch adds a message for the "j" option of dex2oat in order to explicit
its usage.

Change-Id: I5a50e52b38f579f9956ad92a7daaf77bfa4c893f
Signed-off-by: Jean-Philippe Halimi <jean-philippe.halimi@intel.com>
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index dfea783..06a3d3f 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -109,6 +109,11 @@
 
   UsageError("Usage: dex2oat [options]...");
   UsageError("");
+  UsageError("  -j<number>: specifies the number of threads used for compilation.");
+  UsageError("       Default is the number of detected hardware threads available on the");
+  UsageError("       host system.");
+  UsageError("      Example: -j12");
+  UsageError("");
   UsageError("  --dex-file=<dex-file>: specifies a .dex, .jar, or .apk file to compile.");
   UsageError("      Example: --dex-file=/system/framework/core.jar");
   UsageError("");