README: minor tweak to specifically mention GNU Fortran

Lots of people still use GFortran, and lots of people still use
somewhat old versions of it (e.g., if it's bundled in their
older-but-still-installed Linux distros).  So let's specifically
mention it.  This may be a bit overkill, but more specific docs are
usually a Good Thing (i.e., they can prevent questions from being sent
to the mailing list).

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

README

@@ -368,6 +368,12 @@ Compiler Notes
     when Open MPI is built with a Fortran compiler that support the
     INTERFACE keyword and ISO_FORTRAN_ENV.
 
+    *** The Open MPI team has not tested to determine exactly which
+        version of the GNU Fortran compiler suite started supporting
+        what is required for MPI_SIZEOF.  We know that gfortran v4.8
+        (bundled in RHEL 7.x) supports the MPI_SIZEOF interfaces.
+        However, gfortran 4.4 (bundled in RHEL 6.x) does not.
+
   - The level of support provided by the mpi module is based on your
     Fortran compiler.