Added requirement for OpenMP in doc

hpvm/docs/build-hpvm.rst

@@ -25,6 +25,11 @@ Dependencies
 
       * CUDNN 7 is unsupported beyond CUDA 10.2 (starting from CUDA 11)
    
+   * OpenMP (>= 4.0)
+
+      * GCC comes with OpenMP support; OpenMP-4.0 is supported by GCC-4.9 onward.
+        see `here <https://gcc.gnu.org/wiki/openmp>`_ for the OpenMP version supported by each GCC version.
+ 
    * In addition, each version of CUDA-nvcc requires GCC to be not newer than a certain version.
      See `here <https://gist.github.com/ax3l/9489132>`_ for the support matrix.