Modified main README

README.md

 # matVecKernels
 Repository of test cases for matrix vector product
+
+## Repository structure
+* **Data**: folder for matrices data
+* **matVecKernels**: source code for test executables
+* **postProcessor**: utilities for post proceesing test results
+* **preProcessor**: executables to preprocessing data
+* **utils**: utilities to submit test job on HPC systems
\ No newline at end of file