Added some description

Added overview to readme.

README.md

-Benchmarks for NPDet
-====================
-
+EIC Detector Benchmarks
+=======================
 
 [![pipeline status](https://eicweb.phy.anl.gov/jihee.kim/benchmarks/badges/master/pipeline.svg)](https://eicweb.phy.anl.gov/jihee.kim/benchmarks/-/commits/master)
 
+## Overview
+
+Detector benchmarks are meant to test for regressions in individual detector subsystems.
+The analysis is meant to avoid a complex reconstruction. 
+So for now this precludes using [juggler](https://eicweb.phy.anl.gov/EIC/juggler) for processing the events.
+[Physics benchmarks](https://eicweb.phy.anl.gov/EIC/benchmarks/physics_benchmarks) will 
+include the full reconstruction chain processing.
 
 
 ## Adding new benchmarks