Update test readme to include directions about using Makefile.config

hpvm/test/README.md

 # Using HPVM
-Tests are provided, along with a template Makefile for user projects.
+The below benchmarks are provided with HPVM, along with a template Makefile for user projects. In order to be able to build the existing benchmarks, a new `Makefile.config` must be created in [include](/hpvm/test/include) based on the existing `Makefile.config.example`. This configuration file must set up the following paths:
+* LLVM_BUILD_DIR: should point to your local `build` directory of HPVM.
+* CUDA_PATH: should point to your local CUDA installation.
 
 ## Parboil
 Several tests from the [parboil suite](http://impact.crhc.illinois.edu/parboil/parboil.aspx) have been ported to HPVM.