add note about how to add OpenMP support to Xcode on macOS

doc/src/Build_extras.rst

@@ -1929,6 +1929,22 @@ OPENMP package
       For other platforms and compilers, please consult the
       documentation about OpenMP support for your compiler.
 
+.. admonition:: Adding OpenMP support on macOS
+   :class: note
+
+   Apple offers the `Xcode package and IDE
+   <https://developer.apple.com/xcode/>`_ for compiling software on
+   macOS, so you have likely installed it to compile LAMMPS.  Their
+   compiler is based on `Clang <https://clang.llvm.org/>`, but while it
+   is capable of processing OpenMP directives, the necessary header
+   files and OpenMP runtime library are missing.  The `R developers
+   <https://www.r-project.org/>` have figured out a way to build those
+   in a compatible fashion. One can download them from
+   `https://mac.r-project.org/openmp/
+   <https://mac.r-project.org/openmp/>`_.  Simply adding those files as
+   instructed enables the Xcode C++ compiler to compile LAMMPS with ``-D
+   BUILD_OMP=yes``.
+
 ----------
 
 .. _qmmm: