ref #62. Added the user friendly message with USE_OPENMP=1. The users should use...

ref #62. Added the user friendly message with USE_OPENMP=1. The users should use OMP_NUM_THREADS.

When OpenBLAS is compiled with USE_OPENMP=1, it ignores OPENBLAS_NUM_THREADS and GOTO_NUM_THREADS flags.Therefore, you should use OMP_NUM_THREADS.

Without setting OMP_NUM_THREADS, a process will use maximal number of threads on a computing node. Thus, if there are 2 processes on the computing node, the thread will contend against other threads on CPU cores. As a result, the application will hang.