Update package build docs for building the USER-QMMM package

   for a QM/MM simulation.  You must also build Quantum ESPRESSO and

   create a new executable (pwqmmm.x) which links LAMMPS and Quantum

   ESPRESSO together.  These are steps 3 and 4 described in the

   lib/qmmm/README file.  Unfortunately, the Quantum ESPRESSO developers

   have been breaking the interface that the QM/MM code in LAMMPS is using,

   so that currently (Summer 2018) using this feature requires either

   correcting the library interface feature in recent Quantum ESPRESSO

   releases, or using an outdated version of QE. The last version of

   Quantum ESPRESSO known to work with this QM/MM interface was version

   5.4.1 from 2016.

   lib/qmmm/README file.  This requires a compatible Quantum espresso

   and LAMMPS version.  The current interface and makefiles have

   last been verified to work in February 2020 with Quantum Espresso

   versions 6.3 to 6.5.

**CMake build**\ :