README: Add info regarding compiler

README.rst

@@ -182,3 +182,13 @@ Notes
 
 * In order for the algorithm to work the mesh must not be numbered incorrectly
   or disconnected or of somehow degenerate.
+
+* The c++ compiler must have OpenMP installed. This is generally not an issue
+  on g++ or Microsoft Visual Studio. However, in macOS one may need to install
+  llvm from brew in order to use OpenMP: ``brew install llvm``. Then, change the
+  ``CC`` and ``CXX`` environment variables to the following:
+
+  .. code-block:: txt
+
+     CC="/usr/local/opt/llvm/bin/clang -L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"
+     CXX="/usr/local/opt/llvm/bin/clang++ -L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"