Added note about OpenMPI on OP systems

Adds a note to the docs; MS can you please check for accuracy.

doc/RTD/source/GettingStarted/compiling_code.rst

@@ -24,6 +24,15 @@ MPI
 A recent implementation of MPI, such as Open MPI (v2.x or higher), is required,
 or any library that implements at least the MPI 3 standard.
 
+Running SWIFT on OmniPath atchitechtures with Open MPI
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When running SWIFT on an OmniPath system we suggest that Open MPI v3.1.3 or higher
+is used. A bug in the ``psm2`` library causes communications to be lost. It is
+possible to run SWIFT with older versions (tested with v2.1.x) of Open MPI so
+long as ``psm`` is used instead of ``psm2``, i.e. that you invoke ``mpirun``
+with ``--mca btl vader,self -mca mtl psm``.
+
 Libtool
 ~~~~~~~
 The build system depends on libtool.