Add a note about HACCmk to the HACCKernels README

README

@@ -46,5 +46,14 @@ define the preprocessor symbol VERIFICATION_OUTPUT_ONLY when compiling.
 You can enable this option when configuring by passing
 -DVERIFICATION_OUTPUT_ONLY=ON to cmake.
 
+Compared to the older HACCmk procurement benchmark
+(https://asc.llnl.gov/CORAL-benchmarks/#haccmk), this benchmark:
+
+ * More closely matches the parallelization scheme used by the production code.
+ * Uses a more-realistic distribution of interaction-list lengths and
+   out-of-bounds particles.
+ * Includes 4th-, 5th-, and 6th-order kernels.
+
 For more information, contact: Hal Finkel <hfinkel@anl.gov>
 
+