message: Mention mfem.org/building in INSTALL
diff: https://mfem.org
  +This file provides a detailed description of how to build and install the MFEM
  +library. For a simple build, see the step-by-step instructions on the website
  +at https://mfem.org/building.
  +
  The MFEM library has a serial and an MPI-based parallel version, which largely
  share the same code base. The only prerequisite for building the serial version
  of MFEM is a (modern) C++ compiler, such as g++. The parallel version of MFEM
  