README: document some thread support limitations

README

@@ -448,8 +448,8 @@ MPI Functionality and Features
   deprecated_example.c:4: warning: 'MPI_Type_struct' is deprecated (declared at /opt/openmpi/include/mpi.h:1522)
   shell$
 
-- MPI_THREAD_MULTIPLE is supported.  Note that Open MPI must be
-  configured  with --enable-mpi-thread-multiple to get this
+- MPI_THREAD_MULTIPLE is supported with some exceptions.  Note that Open MPI
+  must be configured  with --enable-mpi-thread-multiple to get this
   level of thread safety support.
 
   The following BTLs support MPI_THREAD_MULTIPLE:
@@ -463,6 +463,13 @@ MPI Functionality and Features
   - MXM
   - portals4
 
+  Currently MPI File operations are not thread safe even if
+  MPI is initialized for MPI_THREAD_MULTIPLE support.
+
+  The OpenIB BTL's RDMACM based connection setup mechanism is also
+  not thread safe.  The default UDCM method should be used for applications
+  requiring MPI_THREAD_MULTIPLE support.
+
 - MPI_REAL16 and MPI_COMPLEX32 are only supported on platforms where a
   portable C datatype can be found that matches the Fortran type
   REAL*16, both in size and bit representation.