add short blurb for users about c ABI support

and how to use it

Signed-off-by: Howard Pritchard <[email protected]>

Author: hppritcha

docs/building-apps/index.rst

@@ -14,6 +14,7 @@ Open MPI "wrapper" compilers.
    removed-mpi-constructs
    deprecation-warnings
    building-static-apps
+   mpi-abi
 
 .. warning:: Open MPI does not attempt to interoperate with other MPI
              implementations, nor executables that were compiled for

docs/building-apps/mpi-abi.rst

@@ -0,0 +1,33 @@
+.. _label-building-with-mpi-abi:
+
+Building MPI applications using the MPI ABI
+===========================================
+
+What is the MPI ABI
+-------------------
+
+Starting with MPI 5.0, the MPI standard specifies an ABI for the c and
+Fortran MPI interfaces.  In this release, Open MPI supports the c
+part of the MPI ABI.
+
+By using the MPI ABI, an MPI application can be built against one 
+implementation of MPI that supports the MPI ABI, and later run using
+the MPI ABI compliant MPI library generated using a different MPI
+implementation.  This assumes that the application is dynamically linked.
+
+The MPI 5.0 standard specifies the file name of the MPI ABI compliant
+library - libmpi_abi.  The major version of the library is 1 and minor
+version is 0.
+
+How to build an application using the MPI ABI
+---------------------------------------------
+
+To build an application against the MPI ABI compliant Open MPI library,
+the ``mpicc_abi`` compiler wrapper must be used for compiling and 
+linking the application or shared library.
+
+This release does not support the Fortran ABI so there is no ``mpifort_abi``
+compiler wrapper.  This mixed c/Fortran MPI apps cannot make use of the
+MPI ABI library with this release.
+
+