Adding common README file for benchmarks

llvm/test/VISC/DNN_Benchmarks/README.md

+
+# Directory Structure
+
+* ./benchmarks: code corresponding to each benchmark
+  ./benchmarks/${BENCH_NAME}/bin/approximate.py - script for running autotuner 
+* ./common:
+  ./common/include: include files
+  ./common/bin: path setup files - modify paths to HPVM Build and SRC as per installation
+  
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