Add note about what Paul Hargrove discovered abotu g96 on the NetBSD-6

AMD64 platforms:

    http://www.open-mpi.org/community/lists/devel/2014/01/13748.php

cmr=v1.7.4:reviewer=rhc:subject=README update for g95+NetBSD-6+AMD64

This commit was SVN r30269.

README

@@ -159,6 +159,10 @@ Compiler Notes
   might want to double check that you have the latest bug fixes and
   patches for your compiler.
 
+- Building Open MPI on a NetBSD-6 AMD64 system will run into obscure
+  compile-time failures if f95/g95 is found in the path.  You can work
+  around such problems by removing g95 from your path.
+
 - Absoft 11.5.2 plus a service pack from September 2012 (which Absoft
   says is available upon request), or a version later than 11.5.2
   (e.g., 11.5.3), is required to compile the new Fortran mpi_f08